Thermochemical and Catalytic Conversion Technologies for Future Biorefineries: Volume 1 (Clean Energy Production Technologies) 9811943117, 9789811943119

Table of contents :
Preface
Acknowledgment
Contents
Editor and Contributors
Chapter 1: Scope and Characteristics of the Biomass Sources Suitable for Biorefinery Applications
1.1 Introduction
1.2 Low-Cost Sources of Biomass
1.3 Cost-Effective Cultivation Practices and Impact of Cultivation Condition on the Biomass Characteristics
1.4 Correlation of the Biomass Characteristics with the Biomass Valorization Techniques
1.5 Conclusion and Recommendations
References
Chapter 2: Recalcitrance of Lignocellulosic Biomass and Pretreatment Technologies: A Comprehensive Insight
2.1 Introduction
2.2 Lignocellulosic Biomass (LCB)
2.2.1 Chemical Interactions in Lignocellulosic Biomass
2.3 Components of LCB: Cellulose, Hemicellulose, and Lignin Derivatives
2.3.1 Cellulose: Structure, Synthesis, and Properties
2.3.1.1 Cellulose Structure
2.3.1.2 Chemical Properties of Cellulose
2.3.1.3 Mechanism of Cellulose Synthesis in Plants
2.3.2 Hemicellulose: Structure, Properties, and Synthesis
2.3.2.1 Structure of Hemicellulose
2.3.2.2 Properties of Hemicellulose
2.3.2.3 Hemicellulose Synthesis in Plants
2.3.3 Lignin: Structure, Properties, and Synthesis
2.3.3.1 Structure of Lignin
2.3.3.2 Properties of Lignin
2.3.3.3 Lignin Biosynthesis in Plants
2.4 Recalcitrance of Lignocellulose Biomass (LCB)
2.4.1 Effect of Physical Properties of LCB on Recalcitrance
2.4.1.1 Cellulose Crystallinity
2.4.1.2 Degree of Polymerization (DP)
2.4.1.3 Effect of Pore Volume, Particle Size, and Surface Area
2.4.2 Contribution of Chemical Components of LCB on Recalcitrance
2.4.2.1 Lignin
2.4.2.2 Hemicellulose
2.4.2.3 Proteins in the Cell Wall
2.4.3 Combined Effect of Physical Properties and Chemical Compositions on Recalcitrance
2.5 Pretreatment: Introduction and Its Role
2.6 Pretreatment-Induced Changes on LCB
2.7 Conclusion
References
Chapter 3: Understanding Biomass Recalcitrance: Conventional Physical, Chemical, and Biological Pretreatment Methods for Overc...
3.1 Introduction
3.2 Pretreatment and Hydrolysis of Biomasses: Overview
3.2.1 Physical/Mechanical Pretreatment
3.2.1.1 Milling
3.2.1.2 Irradiation
3.2.1.3 Expansion and Extrusion
3.2.2 Physicochemical Pretreatment
3.2.2.1 Explosion
High-pressure Steam Explosion (HPSE)
Ammonia Fiber Explosion (AFEX)
Carbon Dioxide Explosion
3.2.2.2 Mechanical/Alkaline Pretreatment
3.2.3 Chemical Pretreatment
3.2.3.1 Liquid Hot Water
3.2.3.2 Acid (Weak and Strong Acid Hydrolysis)
Sulfuric Acid
Phosphoric Acid
3.2.3.3 Alkaline Hydrolysis
Sodium Hydroxide
Hydrogen Peroxide
Wet Oxidation
Ozonation
3.2.3.4 Solvent Extraction
3.2.4 Biologicals Pretreatments
3.2.4.1 Fungi
3.2.4.2 Bacteria
3.3 Conclusions
References
Chapter 4: The Pioneering Role of Enzymes in the Valorization of Waste: An Insight into the Mechanism of Action
4.1 Introduction
4.2 Enzymes and Their Characteristics
4.2.1 Enzyme-Substrate Interaction
4.2.2 Enzyme Thermodynamics and Kinetics
4.3 Chemical and Structural Factors Guiding Enzyme Hydrolysis
4.3.1 Composition of Lignocellulosic Biomass
4.3.1.1 Cellulose
4.3.1.2 Hemicellulose
4.3.1.3 Lignin
4.3.2 Starch
4.4 Enzymes and their Application in Waste
4.4.1 Hydrolytic Enzymes Involved in the Valorization of Lignocellulosic Waste
4.4.1.1 Cellulases
4.4.1.2 Hemicellulases
4.4.1.3 Lignin-Degrading Enzymes
4.4.2 Hydrolytic Enzymes Involved in the Valorization of Food Waste
4.4.3 Hydrolytic Enzymes in Biodegradation and Valorization of Non-biodegradable Plastics Waste
4.5 Recent Biotechnological Trends in Increasing the Efficacy of Enzymes in the Waste Valorization
4.5.1 Techniques to Decipher Newer Biocatalyst Candidates
4.5.2 Isolation of Enzymes from Extremophiles
4.5.3 Genetic Engineering or Recombinant DNA Technology for the Production of Recombinant Protein in a Microbial Host
4.5.4 Immobilization of Enzymes
4.5.5 Cell Surface Engineering
4.6 Conclusions
References
Chapter 5: Thermochemical Conversion of Biomass into Value-Added Materials for Effluent Treatment Applications
5.1 Introduction
5.2 Biomass as a Source of Fuel
5.3 Value-Added Pathway from Biomass for Different Applications
5.4 An Alternate Strategy for the Utilization of Waste Biomass
5.5 Biomass for Effluent Treatment Processes
5.6 Application of Raw Biomass in the Effluent Treatment Process
5.7 Biomass-Derived Activated Carbon for the Effluent Treatment Process
5.8 Biomass-Based Composite Materials for Effluent Treatment
5.8.1 Biomass-Based Magnetic Materials for Effluent Treatment
5.8.2 Biomass-Based Polymer and Clay Composites for Effluent Treatment
5.8.3 Biomass-Based Graphene Composites for Effluent Treatment
5.8.4 Biomass-Based Metal Oxides Composites such as Catalyst and Catalyst Supports
5.9 Conclusions
References
Chapter 6: Cellulase: A Catalytic Powerhouse for Lignocellulosic Waste Valorisation
6.1 Introduction
6.2 Recent Advances in Cellulase Production and Purification
6.2.1 Microbial Production of Cellulase
6.2.1.1 Solid-State Fermentation (SSF)
6.2.1.2 Submerged Fermentation (SmF)
6.3 Immobilisation and Molecular Approaches for Enhanced Cellulolytic Action
6.4 Role of Cellulase in Biorefinery Application
6.4.1 Consolidated Bioprocessing for Biorefinery
6.4.2 Cellulases in Agro-industrial Waste Biorefinery
6.4.3 Challenges of Cellulase-Based Biorefinery
6.5 Role of Cellulase in Value-Added Product Formation
6.5.1 Lactic Acid
6.5.2 Succinic Acid
6.5.3 Gluconic Acid
6.5.4 Citric Acid
6.5.5 Microbial Polymers
6.5.5.1 Polyhydroxyalkanoates
6.5.5.2 Nanocellulose
6.6 Role of Cellulase in a Circular Economy
6.6.1 Valorisation via Carbon-Neutral Technologies
6.6.2 Life Cycle Assessment of Cellulase-Based Biorefinery
6.6.3 Increased Circularity via Onsite Application
6.6.4 Challenges and Perspectives
6.7 Conclusion
References
Chapter 7: Technical Criteria for Converting Biomass to High Liquid Bio-Oil Yields
7.1 Introduction
7.2 Pyrolysis
7.2.1 Different Routes of Pyrolysis
7.2.1.1 Slow Pyrolysis
7.2.1.2 Intermediate Pyrolysis
7.2.1.3 Fast Pyrolysis
7.2.1.4 Flash Pyrolysis
7.2.2 Technical Specifications: Pretreatment, Characterization, and Mechanism
7.2.2.1 Pretreatment
Physical Treatment
Chemical Pretreatment
Biological Pretreatment
7.2.3 Feedstock Drying
7.2.3.1 Characteristics of Feedstocks
7.2.3.2 Biomass Characterization
7.3 Future Prospects
7.4 Summary
References
Chapter 8: Biomass Pyrolysis and its Multiple Applications
8.1 Introduction
8.2 Pyrolysis
8.2.1 Physics of Pyrolysis
8.2.2 Types of the Pyrolysis Process
8.2.2.1 Slow Pyrolysis
8.2.2.2 Fast Pyrolysis
8.2.2.3 Flash Pyrolysis
8.2.2.4 Intermediate Pyrolysis
8.3 Effects of Process Parameters
8.3.1 Process Temperature
8.3.2 Heating Rate
8.3.3 Residence Time
8.3.4 Particle Size
8.4 Biochar Activation
8.4.1 Physical Activation
8.4.2 Chemical Activation
8.5 Biochar Applications
8.5.1 Biochar as an Absorbent
8.5.1.1 Biochar for Wastewater Treatment
Biochar for Heavy Metal Removal
Removal of Organic Pollutants
8.5.1.2 Biochar for Air Purification
8.5.2 Biochar as a Soil Amendment
8.5.3 Biochar as a Catalyst
8.5.4 Biochar as an Alternative to Fuel
8.5.5 Biochar as an Additive
8.5.5.1 Biochar Used as an Additive in the Construction Sector
8.5.5.2 Biochar as an Additive in Composting
8.5.6 Other Modern Applications
8.6 Conclusion
References
Chapter 9: Recent Advances in Direct Catalytic Thermochemical Gasification of Biomass to Biofuels
9.1 Introduction
9.2 Biomass Gasification
9.2.1 Gasification Chemistry
9.3 Gasifying Agents
9.4 Types of Gasifiers
9.4.1 Fixed Bed Gasifier
9.4.1.1 Updraft Gasifier
9.4.1.2 Downdraft Gasifier
9.4.1.3 Crossdraft Gasifier
9.4.2 Fluidized Bed Gasifier
9.4.2.1 Bubbling Fluidized Bed Gasifier
9.4.2.2 Circulating Fluidized Bed Gasifier
9.4.3 Entrained Flow Gasifier
9.4.4 Plasma Gasifier
9.5 Tar
9.5.1 Composition
9.5.2 Effects of Operating Conditions on Tar Yield
9.5.3 Application Limits
9.6 Effect of Key Operating Parameters on Gasification
9.6.1 Biomass/Feedstock
9.6.2 Temperature
9.6.3 Pressure
9.6.4 Residence Time
9.6.5 Equivalence Ratio
9.6.6 Steam to Biomass Ratio
9.7 Need for Catalytic Biomass Gasification
9.8 Types of Catalysts
9.8.1 Natural Mineral Catalysts
9.8.1.1 Dolomite
9.8.1.2 Limonite
9.8.1.3 Olivine
9.8.1.4 Calcite and Magnesite
9.8.2 Alkali Metal Catalysts
9.8.3 Metal-Based Catalysts
9.8.3.1 Ni-Based Catalysts
9.8.3.2 Fe-Based Catalysts
9.8.3.3 Metal and Metal Oxide Catalysts
9.8.4 Zeolites
9.8.5 Activated Char Catalysts
9.9 Comparison of Different Catalysts
9.9.1 General Comparison
9.9.2 Comparison of Gas Yield and Tar Conversion
9.9.2.1 Natural Mineral-Based Catalysts
9.9.2.2 Metal-Based Catalysts
9.10 Catalytic and Deactivation Mechanisms
9.10.1 Catalytic Mechanism
9.10.1.1 Reforming Mechanism of Real Tar Using Char Catalyst
9.10.1.2 Steam Reforming Mechanism of Tar Model (Toluene)
9.10.2 Deactivation Mechanisms
9.10.2.1 Coking or Coke Formation
9.10.2.2 Sintering
9.10.2.3 Sulfur Poisoning
9.11 Products and Application of Biomass Gasification
9.11.1 Products
9.11.1.1 Bio-syngas
9.11.1.2 Heat and Electricity Generation
9.11.1.3 Methanol
9.11.1.4 Ammonia Synthesis
9.11.1.5 Ethanol
9.11.1.6 Gasoline
9.11.1.7 Diesel
9.11.1.8 Methane
9.11.1.9 Hydrogen
9.11.2 Applications
9.11.2.1 Boilers
9.11.2.2 Gas Turbines
9.11.2.3 Internal Combustion Engines
9.11.2.4 Fuel Cells
9.11.2.5 Fischer-Tropsch Synthesis
9.12 Recent Advances and Future Perspectives of Catalytic Biomass Gasification
9.12.1 Recent Advances
9.12.2 Future Perspectives
9.13 Summary
References
Chapter 10: Recent Advances in Fast Pyrolysis and Oil Upgradation
10.1 Introduction
10.2 Bio-oil
10.2.1 Feedstock for Bio-Oil
10.2.1.1 Lignin, Cellulose, and Forest Residue
10.2.1.2 Bark and Wood Trunk
10.2.1.3 Nutshell Waste
10.2.1.4 Seeds
10.2.1.5 Agricultural Residue
10.2.1.6 Algae
10.2.1.7 Animal Waste
10.2.1.8 Other Feedstocks
10.2.2 Bio-oil from Pyrolysis
10.2.2.1 Slow Pyrolysis
10.2.2.2 Steam Pyrolysis and Vacuum Pyrolysis
10.2.2.3 Hydro-Pyrolysis and Methano-Pyrolysis
10.2.2.4 Hydrothermal Carbonization, Liquefaction, and Gasification
10.2.2.5 Torrefaction
10.2.2.6 Fast Pyrolysis
10.2.2.7 Flash Pyrolysis
10.2.2.8 Vacuum Flash Pyrolysis and Flash-Hydro-Pyrolysis
10.2.2.9 Microwave-Assisted Pyrolysis
10.2.2.10 Solar-Assisted Fast or Flash Pyrolysis
10.2.2.11 Plasma Pyrolysis
10.2.2.12 Gasification
10.2.3 Physicochemical Properties
10.3 Fast Pyrolysis Reactor/Pyrolyzer
10.3.1 Fixed Bed Reactor
10.3.2 Fluidized Bed Reactor
10.3.2.1 Bubbling Fluidized Bed Reactor (BFBR)
10.3.2.2 Circulating Fluidized Bed Reactor (CFBR)
10.3.2.3 Conical Spouted Bed Reactor (CSBR)
10.3.2.4 Internally Circulating Fluidized Bed Reactor (ICFBR)
10.3.2.5 Rapid Fluidized Bed Reactors
10.3.3 Ultra-Rapid Pyrolyzer
10.3.4 Free-Fall Reactor
10.3.5 Rotating Cone Pyrolyzer
10.3.6 Ablative Pyrolyzer
10.3.6.1 Vortex Reactor
10.3.6.2 Rotating Disk Reactor
10.3.7 Auger Reactor or Moving Bed Reactor
10.3.8 PyRos Reactor
10.3.9 Multiple Hearth or Vacuum Pyrolysis Reactors
10.3.10 Microwave Reactor
10.3.11 Solar Reactor
10.3.12 Plasma Reactor
10.3.13 Pulse Shock Tube
10.4 Enhancing Bio-Oil Properties
10.4.1 Bio-Oil Pretreatment Methods
10.4.2 Upgradation Technique
10.4.2.1 Catalytic Pyrolysis
10.4.2.2 Hydroprocessing
10.4.2.3 Steam Reforming
10.4.2.4 Supercritical Fluids Extraction
10.4.2.5 Esterification
10.4.2.6 Emulsification and Blending
10.4.2.7 Molecular Distillation
10.4.2.8 Electrochemical or Electrocatalytic Upgrading of Bio-oil
10.4.3 Applications of Bio-oil
10.5 Techno-Economics and Environmental Sustainability of Bio-oil
10.6 Conclusion
References
Chapter 11: An Overview on Organosolv Production of Bio-refinery Process Streams for the Production of Biobased Chemicals
11.1 Bio-refineries
11.1.1 Processing for the Bio-Based Economy: Bio-refineries
11.2 Biomass Feedstocks
11.3 The Full-Scale Operation of the Bio-refinery Concept
11.4 Biobased Chemicals
11.4.1 Cost and Performance of Biobased Chemicals
11.5 Biomass Pre-treatment´s Objectives
11.5.1 Lignocellulosic Biomass Physical and Chemical Characteristics
11.6 LCB Pre-treatment Technologies
11.6.1 Mechanical Comminution
11.6.2 Steam or Water Vapor Explosion
11.6.3 Liquid Hot Water Pre-treatment (LHW)
11.6.4 Ammonia Fiber Explosion (AFEX)
11.6.5 CO2 Explosion Pre-treatment
11.6.6 Wet Oxidation Pre-treatment (WOP)
11.6.7 Acid Hydrolysis
11.6.8 Peroxyformic Acid
11.6.9 Alkaline Hydrolysis
11.6.10 Ozonolysis
11.6.11 Organosolv Pre-treatment
11.6.12 Biological Pre-treatment
11.7 Organosolv Technology Outperforms Other Pre-treatment Methods
11.7.1 Different Solvents for Organosolv Pulping
11.8 Overview of Organosolv Pre-treatment
11.9 The Chemistry of Organosolv Delignification
11.9.1 The Nature of Organosolv Pulping
11.10 Advantages and Disadvantages of Organosolv Pre-treatment
11.11 Organosolv Pre-treatment for Biofuel Production
11.11.1 Organosolv: A Potential Pre-treatment Technology for Bioethanol Production
11.12 Conclusions
References

Citation preview

Clean Energy Production Technologies Series Editors: Neha Srivastava · P. K. Mishra

Pradeep Verma   Editor

Thermochemical and Catalytic Conversion Technologies for Future Biorefineries Volume 1

Clean Energy Production Technologies Series Editors Neha Srivastava, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India P. K. Mishra, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India

The consumption of fossil fuels has been continuously increasing around the globe and simultaneously becoming the primary cause of global warming as well as environmental pollution. Due to limited life span of fossil fuels and limited alternate energy options, energy crises is important concern faced by the world. Amidst these complex environmental and economic scenarios, renewable energy alternates such as biodiesel, hydrogen, wind, solar and bioenergy sources, which can produce energy with zero carbon residue are emerging as excellent clean energy source. For maximizing the efficiency and productivity of clean fuels via green & renewable methods, it’s crucial to understand the configuration, sustainability and technoeconomic feasibility of these promising energy alternates. The book series presents a comprehensive coverage combining the domains of exploring clean sources of energy and ensuring its production in an economical as well as ecologically feasible fashion. Series involves renowned experts and academicians as volume-editors and authors, from all the regions of the world. Series brings forth latest research, approaches and perspectives on clean energy production from both developed and developing parts of world under one umbrella. It is curated and developed by authoritative institutions and experts to serves global readership on this theme.

Pradeep Verma Editor

Thermochemical and Catalytic Conversion Technologies for Future Biorefineries Volume 1

Editor Pradeep Verma Department of Microbiology, Bioprocess & Bioenergy Lab Central University of Rajasthan, Bandarsindri, Ajmer, Kishangarh, Rajasthan, India

ISSN 2662-6861 ISSN 2662-687X (electronic) Clean Energy Production Technologies ISBN 978-981-19-4311-9 ISBN 978-981-19-4312-6 (eBook) https://doi.org/10.1007/978-981-19-4312-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Dedicated to my beloved mother

Preface

The rise in global energy needs and fast depletion of the existing conventional fossil fuels force the stakeholders to search for alternatives. Biomass such as food crops, lignocellulosic, algal, and hybrid energy crops can be converted to biofuel and different value-added chemicals, thus acting as a potential substitute for petroleum-based biorefineries. Also, this strategy helps to overcome the pollution caused by the management of this waste generated worldwide. The widespread availability and structural composition of biomass motivated the scientific community to look at it as a potential substrate for the oil and chemical industry. Thus, leading to the development of biomass-based biorefinery. However, the conversion of biomass to high-value products is not as easy as it seems. It involves several stages such as pretreatment, hydrolysis, fermentation, and recovery. The pretreatment and hydrolysis of biomass involve several thermochemical and advanced catalytic technologies. This book will attempt to provide an account of knowledge on biomass available for biomass-based biorefineries. It focuses on understanding the recalcitrance of biomass and how it limits the overall conversion efficiency. It also gives an insight into different conventional approaches available for pretreatment and hydrolysis of the biomass. The chapter deals with highlights on how enzymes can be a powerhouse and play pioneering roles in biomass valorization. The book will also throw light on technical aspects of thermochemical conversion strategies such as pyrolysis, gasification, and organosolv methods. Further the generation of value-added materials such as high-quality bio-oil, biochars, and biobased chemicals are also included in the book. These high-value compounds can be put to widespread application in biofuel, biocatalyst, waste bioremediation (heavy metal removal), air purification, and effluent treatment applications. The book will also provide literature on the limitations of already existing technologies and prospects of each technology. This book is of interest to teachers, researchers, bioenergy scientists, capacity builders, and policymakers. Also, the book serves as an additional reading

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material for undergraduate and graduate students of energy studies, chemical engineering, biotechnology, and environmental sciences. National and international energy scientists and policymakers will also find this book to be a useful read. Ajmer, Kishangarh, Rajasthan, India

Pradeep Verma

Acknowledgment

First, I would like to convey my gratitude to the series editor Dr. Neha Srivastava and Prof. P.K. Mishra considering the submission of this book entitled Thermochemical and Catalytic Conversion Technologies for Future Biorefineries-Vol I under the book series Clean Energy Production Technologies. I am thankful to Springer Nature for accepting my proposal to act as editor for this book volume. This volume of the book series is only possible because of the support from all the researchers and academicians who contributed to the book; therefore, the editor is thankful for their contribution. I would also like to thank my Ph.D. scholar, Dr. Bikash Kumar, currently working as a Post-doctoral Researcher at IIT Guwahati for providing me with all the necessary technical support and editorial assistance during the entire stage of book development. I am also thankful to the Central University of Rajasthan (CURAJ), Ajmer, India, for providing infrastructural support and a suitable teaching and research environment. The teaching and research experience at CURAJ has provided the necessary understanding of the needs of academicians, students, and researchers in a book that was greatly helpful during the development of the book. I am always thankful to God and my parents for their blessings. I also express my deep sense of gratitude to my wife Savita and my son Mohak and daughter Netra for their support during the development of the book and in life.

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Contents

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Scope and Characteristics of the Biomass Sources Suitable for Biorefinery Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Azeem Asghar, Muhammad Afzal, Rizwan Haider, Muhammad Sajjad Ahmad, and Muhammad Aamer Mehmood Recalcitrance of Lignocellulosic Biomass and Pretreatment Technologies: A Comprehensive Insight . . . . . . . . . . . . . . . . . . . . . Rohit Rai, Vikash Kumar, and Prodyut Dhar Understanding Biomass Recalcitrance: Conventional Physical, Chemical, and Biological Pretreatment Methods for Overcoming Biomass Recalcitrance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saurabh Kumar, Richa Prasad Mahato, Kuldeep Gupta, Pritam Bardhan, Muzamil Ahmad Rather, Manabendra Mandal, and Rupam Kataki The Pioneering Role of Enzymes in the Valorization of Waste: An Insight into the Mechanism of Action . . . . . . . . . . . . . . . . . . . . Anupama Binoy, Revathy Sahadevan, Suchi Chaturvedi, and Sushabhan Sadhukhan

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Thermochemical Conversion of Biomass into Value-Added Materials for Effluent Treatment Applications . . . . . . . . . . . . . . . . 125 Nethaji Sundarabal, Vairavel Parimelazhagan, Suganya Josephine Gali Anthoni, Praveen Kumar Ghodke, and Sivasamy Arumugam

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Cellulase: A Catalytic Powerhouse for Lignocellulosic Waste Valorisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Sukhendra Singh, Ipsita Chakravarty, Shankar Mukundrao Khade, Jyoti Srivastava, and Rupika Sinha

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Technical Criteria for Converting Biomass to High Liquid Bio-Oil Yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Naval Koralkar and Praveen Kumar Ghodke

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Biomass Pyrolysis and its Multiple Applications . . . . . . . . . . . . . . . 205 Shivangi Pathak, Anil Kumar Sakhiya, and Priyanka Kaushal

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Recent Advances in Direct Catalytic Thermochemical Gasification of Biomass to Biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Rupesh S. and Deepanraj

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Recent Advances in Fast Pyrolysis and Oil Upgradation . . . . . . . . . 297 Sameer Ahmad Khan, Dushyant Kumar, Subodh Kumar, Adya Isha, Tinku Casper D’Silva, Ram Chandra, and Virendra Kumar Vijay

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An Overview on Organosolv Production of Bio-refinery Process Streams for the Production of Biobased Chemicals . . . . . . . . . . . . . 345 Veluru Sridevi, Dadi V. Suriapparao, Hemanth Kumar Tanneru, and K. S. N. V. Prasad

Editor and Contributors

About the Editor Pradeep Verma is a Professor at the Department of Microbiology, School of Life Sciences, CURAJ, Ajmer, India. He worked as an Associate Professor and Reader-Founder Head at GGV, Bilaspur and Assam University, Silchar. He received his Ph.D. from Sardar Patel University, Gujarat and his Master’s and Bachelor’s from MDS University, Rajasthan. He worked as visiting scientist at UFZ Centre for Environmental Research, Halle, Germany, and a Postdoctoral Fellow at ASCR Prague, Gottingen University, Germany, and RISH Kyoto University Japan. He also worked at Reliance Life Sciences, Mumbai, India. His stay at Japan and Reliance Life Sciences resulted in several commercialized National and International patents. He is a recipient of various prestigious awards and fellowships such as the Ron Cockcroft Award by Swedish society, UNESCO Fellow, DFG fellow, and JSPS PDF. He is a fellow of the Mycological Society of India and the Biotech Research Society of India. His area of expertise involves microbial diversity, bioremediation, bioprocess development, lignocellulosic, and algal biomass-based biorefinery. He has contributed to 71+ research papers, 40+ book chapters, and 11 edited books. He is an editorial board member and reviewer of several prestigious high impact journals.

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Editor and Contributors

Contributors Muhammad Afzal Soil and Environmental Biotechnology Division, National Institute for Biotechnology & Genetic Engineering, Faisalabad, Punjab, Pakistan Muhammad Sajjad Ahmad Tianjin Key Laboratory of Clean Energy and Pollution Control, School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, China Azeem Asghar Bioenergy Research Center, Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Pritam Bardhan Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Tezpur, Assam, India Anupama Binoy Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Ipsita Chakravarty Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India Ram Chandra Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Suchi Chaturvedi Department of Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Indore, Madhya Pradesh, India Tinku Casper D’ Silva Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Deepanraj Department of Mechanical Engineering, Jyothi Engineering College, Thrissur, Kerala, India Prodyut Dhar School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Praveen Kumar Ghodke Department of Chemical Engineering, National Institute of Technology Calicut, Kozhikode, Kerala, India Kuldeep Gupta Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Tezpur, Assam, India Rizwan Haider Institute of Energy & Environmental Engineering, University of the Punjab, Lahore, Punjab, Pakistan Adya Isha Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India G. A. Suganya Josephine Department of Chemistry, Aarupadai Veedu Institute of Technology—Vinayaka Mission Research Foundation, Rajiv Gandhi Salai, Paiyanoor, Kanchipuram, Tamil Nadu, India

Editor and Contributors

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Rupam Kataki Department of Energy, Tezpur University (A Central University), Napaam, Tezpur, Assam, India Priyanka Kaushal Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Shankar Mukundrao Khade School of Engineering, Ajeenkya D Y Patil University, Pune, Maharashtra, India Sameer Ahmad Khan Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Naval Koralkar Department of Chemical Engineering, ITM (SLS) Baroda University, Vadodara, Gujarat, India Dushyant Kumar Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Saurabh Kumar Bioprospection and Product Development Division, CSIR-Central Institute of Medicinal and Aromatic Plants, Lucknow, Uttar Pradesh, India Subodh Kumar Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Vikash Kumar School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Richa Prasad Mahato Department of Microbiology, Kanya Gurukul Campus, Gurukul Kangri University, Haridwar, Uttarakhand, India Manabendra Mandal Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Tezpur, Assam, India Muhammad Aamer Mehmood Bioenergy Research Center, Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan S. Nethaji Department of Chemical Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India Shivangi Pathak Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India K. S. N. V. Prasad Department of Chemical Engineering, College of Engineering (A), Andhra University, Visakhapatnam, Andhra Pradesh, India Rohit Rai School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Muzamil Ahmad Rather Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Tezpur, Assam, India

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Rupesh S Department of Mechanical Engineering, PES College of Engineering, Mandya, Karnataka, India Sushabhan Sadhukhan Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Department of Biological Sciences and Engineering, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Physical & Chemical Biology Laboratory, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Revathy Sahadevan Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Anil Kumar Sakhiya Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India Sukhendra Singh Department of Biotechnology, Dr B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India Rupika Sinha Department of Biotechnology, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, Uttar Pradesh, India A. Sivasamy Catalysis Science Laboratory, CSIR-Central Leather Research Institute (CSIR-CLRI), Chennai, Tamil Nadu, India Veluru Sridevi Department of Chemical Engineering, College of Engineering (A), Andhra University, Visakhapatnam, Andhra Pradesh, India Jyoti Srivastava Department of Biotechnology, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, Uttar Pradesh, India Dadi V. Suriapparao Department of Chemical Engineering, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Hemanth Kumar Tanneru Department of Chemical Engineering, Indian Institute of Crude Oil and Energy, Visakhapatnam, Andhra Pradesh, India P. Vairavel Department of Chemical Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India Virendra Kumar Vijay Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India

Chapter 1

Scope and Characteristics of the Biomass Sources Suitable for Biorefinery Applications Azeem Asghar, Muhammad Afzal, Rizwan Haider, Muhammad Sajjad Ahmad, and Muhammad Aamer Mehmood

Abstract Biomass-derived biorefineries seem a promising approach for the complete valorization of biomass into bioenergy and a range of bioproducts. However, the product quality and bioprocessing strategy highly depend on the nature, composition, and quality of the biomass feedstock. The source and cultivation conditions do not only affect the quality and composition of the biomass but also affect the cost of biomass. Therefore, the choice of cultivation conditions and selection of biomass suitable for its subsequent use is critically important. Accordingly, residual biomass from agricultural or industrial activities or biomass production on marginal lands using wastewater offers an opportunity to produce low-cost biomass without creating any competition with food or land for food. However, it is important to consider that how these conditions affect the nature and composition of biomass with reference to its downstream processing. This book chapter covers the desired characteristics of the biomass to consider it as a potential feedstock for biorefinery while achieving environmental and economic sustainability.

A. Asghar · M. A. Mehmood (*) Bioenergy Research Center, Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan e-mail: [email protected] M. Afzal National Institute for Biotechnology and Genetic Engineering College, Pakistan Institute of Engineering and Applied Sciences (NIBGE-C, PIEAS), Faisalabad, Pakistan R. Haider Institute of Energy & Environmental Engineering, University of the Punjab, Lahore, Punjab, Pakistan M. S. Ahmad Tianjin Key Laboratory of Clean Energy and Pollution Control, School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, China © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_1

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Keywords Feedstock characteristics · Environmental sustainability · Bioenergy · Biorefinery

Abbreviation GHG

1.1

Greenhouse gases emissions

Introduction

Urbanization, industrialization, and transportation depend on fossil resources to meet their energy needs (Tey et al. 2021), but their depletion and negative impacts on the environment face challenges in terms of both social and economic impacts (Liu et al. 2021). A strong global dependence and huge consumption of petroleum derivatives are causing political and environmental concerns due to greenhouse gases emissions (GHG) like nitrogen oxides, methane, and carbon dioxide (NOX, CH4, CO2). Accordingly, there has been a rush towards finding alternative resources to fuel the current and rising future needs of liquid fuels. It is believed that biomass is one of the most promising sources, due to its renewable and sustainable nature and is considered a third fuel source used to generate bioenergy, chemical, and industrial products. Lignocellulosic biomass (the second-generation feedstock) has shown maximum accessibility in subtropical, and tropical macroclimates worldwide with an annual production of 1.3 billion tons, whereas only 3% of it is being used to produce biofuels, bioenergy, and biochemicals (Baruah et al. 2018). Biomass-derived biorefineries are believed to be an encouraging path to develop a new bio-industry by using the plants’ biomass for gaining the food, non-toxic and pure feed, and materials for the chemical industries, energy, and other valuable products (Sanna et al. 2016). The selection of the raw material for biorefinery primarily is influenced by the origin and nature of the feedstock. Cultivation conditions affect the composition of biomass which subsequently determines the process parameters for the downstream processing of the biomass. Instead of using freshwater and agricultural lands, biomass for biorefineries can be produced using non-arable lands and urban wastewater. Likewise, integration of the biomass production landscapes with the phytoremediation using floating gardens or constructed wetlands could be a promising green approach to produce biomass as a feedstock for the biorefinery (Zhang et al. 2020). This chapter is focused on the impact of biomass source, its composition, and nature for its utilization as a biorefinery feedstock. Biorefinery is a multidisciplinary approach that involves the transformation of biomass through sustainable processing to produce biofuels and value-added chemicals with a zero or minimum waste generation, where each by-product of the process is believed to be recycled to ensure environmental sustainability and

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cost-effectiveness. The biorefinery approach is a range of combined techniques targeting to complete suitable conversion of biomass into their value-added products such as hydrogen, carbohydrates, and proteins, with the simultaneous production of biofuel, bioenergy, and biochemical (Bhardwaj et al. 2021; Carvalho et al. 2014; Kumar and Verma 2021; Verma 2020). Biorefineries need prime conversion efficiency of lignocellulosic biomass, within efficient production of energy (Hirani et al. 2018). The European vision of biorefinery 2030 document entitled supply of the tools and data to enable policymakers to draw a framework for the development of sustainable research based, with a network of refinery playing a vital role (Płaza and Wandzich 2016). Several biorefinery systems are believed to be developed in the future by taking the advantage of modern flexible technologies for efficient biotransformation of biomass into myriads of green products (Panwar et al. 2011). With respect to the technology employed, biorefineries can be categorized as: (i) pyrolysis-based refinery, (ii) fermentation-based refinery, (iii) hydrothermalbased refinery, and (iv) bio syngas-based refinery. All of these are used to produce materials, biofuels, and chemicals from the biomass.

1.2

Low-Cost Sources of Biomass

Production of biomass on agricultural land in huge quantities is a big challenge at present due to scarcity of the fertile soils so marginal lands can be employed for this purpose and instead of freshwater, the bioenergy crops can be cultivated using urban wastewater (Asghar et al. 2021) which offers additional benefits of phytoremediation and mitigating atmospheric carbon (Kurade et al. 2021). Wastewater is often rich in nutrients, where no additional sources (e.g., fertilizers) are needed to achieve higher biomass productivity. Besides, it does not only reduce the cost of cultivation but also does not cause any competition with the freshwater sources and agricultural lands. Another possibility is the integration of biomass production facilities with the wastewater treatment plants. For instance, para grass was cultivated on industrial and urban wastewater using a floating garden system, and it was found that overall biomass productivity was better when compared to the para grass biomass produced on the soil (Asghar et al. 2021; Singh et al. 2022; Agrawal and Verma 2021). Similarly, salt-affected soil was used to produce biomass in huge quantities without any fertilizers, fungicides, or pesticides with low cost and ensure environmental sustainability. The biomass of Brachiaria mutica (Asghar et al. 2021), Typha domingensis, Phragmites australis, and Leptochloa fusca (Afzal et al. 2019b) has been cultivated on wastewater and Parthenium hysterophorus, Pennesetum benthiumo (Ahmad et al. 2019), Calotropis procera (Ahmad et al. 2021), Urochloa mutica (Ahmad et al. 2017), and Cymbopogon schoenanthus (Mehmood et al. 2017b) have been cultivated on salt-affected soils. The biomass productivities of these grasses on poor soils have shown a promising potential of these soils to produce low-cost biomass (Fig. 1.1).

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Fig. 1.1 Global sources of biomass sources for bioenergy production

Marine waste Agricultural Crops & Residues

Industrial Residues

Biomass Sources Animal Residues

Sewage

Forestry Crops & Residues

Municipal Solid waste

Table 1.1 Different lignocellulosic biomass and their composition Biomass Pearl millet Calotropis procera Ricinus communis Sagwan sawdust Rice husk Sunflower disc

Extractives (%) 4.26

Hemicellulose (%) 23.16

Lignin (%) 15.75

Cellulose (%) 48.93

32.5

33

17.91

16.57

References Laouge and Merdun (2020) Ahmad et al. (2021)

16.40

22.40

20.20

38.42

Kaur et al. (2018)

11.3

12.9

24.7

51.1

17.16 14.12

26.26 27.87

33.10 31.46

Gupta and Mondal (2019) Ashraf et al. (2019) Ashraf et al. (2019)

Biomass is either classified according to its composition/characteristics (Table 1.1) or the source (Table 1.2). Low-cost biomass can be obtained through agricultural practices, residues from forestry, industrial wastes, aquaculture, or municipal waste (Poveda-Giraldo et al. 2021). It is estimated that global plant biomass production is around 200  109 metric tons where 90% of the biomass is lignocellulosic in nature (Saini et al. 2015). Lignocellulosic biomass mainly comprises three components including cellulose (35–50%), hemicellulose (20–35%), and lignin (14–26%) with a small percentage of extractives. Macromolecules gather to form 3-D polymorphic structures which represent their function to deliver skeleton material with stiffness to the cell wall of plants (Madanayake et al. 2017). The main part of lignocellulosic material is hemicellulose, cellulose converted into

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Table 1.2 Sources and products of biomass feedstocks First generation Rich sugar biomass Sources Sugar beet, sugarcane, starch, vegetable oils Products Biogases, bio-oils, and biofuels

Second generation Lignocellulosic biomass

Third generation Aquatic biomass

Fourth generation Rich solar-efficient aquatic biomass

Non-food crops wheat straw, maize stovers, corn, wood, solid waste

Algae, microalgae, cyanobacteria

Genetically engineered Plants and algal materials

Biochemicals, electricity, power, heat, biogases, bio-oils, and biofuels

Fertilizers, biochemicals, electricity, power, heat, biogases, bio-oils, and biofuels

Biogases, bio-oils and biofuels, fertilizers, biochemicals, electricity, power, and heat

monosaccharides unit by a process of depolymerization such as xylose, glucose along mannose, galactose, sugar acids (galacturonic and glucuronic acid), and arabinose. In biomass, the matrix of hydrogen and covalent bond connect the lignin and hemicellulose with cellulose and also observed that lignin due to its strong chemical bonding nature also plays a role as binding agent (Kassaye et al. 2017).

1.3

Cost-Effective Cultivation Practices and Impact of Cultivation Condition on the Biomass Characteristics

Wastewater and marginal lands have a great economic impact in terms of cost used to produce biomass because these both resources it does not use agricultural land and have no negative effect but in addition, it makes more profitable because the use of non-agriculture land and eliminate the toxic, heavy metal from wastewater which directly improve the bioenergy production. Biomass cultivation in wastewater bodies used a novel approach with a very low cost called a floating wetland system (Afzal et al. 2019a; Asghar et al. 2021; Goswami et al. 2021a). Marginal lands and wastewater which are not used for agriculture purposes can be used to produce cost-effective biomass without posing any negative impact on the environment. Municipal wastewater and industrial wastewater are serious global environmental problems, so a new phytoremediation treatment of wastewater is employed for the cultivation of biomass using a floating gardening system. So, this approach is a three-in-one approach offering wastewater treatment, fixation of atmospheric carbon, and carbon-neutral biomass production. The potential of poor soils for the costeffective production of biomass is shown in Table 1.3.

Perennial, adapted to Mediterranean environment

Circumpolar subarctic and cool temperate

Perennial, adapted to temperate regions, wet-soils, colder climatic, flood plains

Giant reed (Arundo donax L.)

Poplar (Populus tremula)

Reed canary grass (Phalaris arundinacea L.)

Bamboo (Bambusa balcooa) Cardoon (Cynara cardunculus) Eucalyptus (Eucalyptus obliqua)

Phalaris arundinacea

Nature, adapted climate Perennial aquatic plant native to tropical desert to the subtropical or warm temperate desert to rainforest zones P. arundinacea now has a world-wide distribution. It is regarded as native to both North America and Eurasia Perennial, adapted to moderately acidic loamy soils, warm humid environment Perennial, adapted to high temperature and low rainfall Perennial, adapted to temperate, tropical, and subtropical poor soils

Crop Eichhornia crassipes

Table 1.3 Low-cost biomass sources for bioenergy

15

6–15.8

36

20

7.4–14.6

40–50

35–45

Biomass Yield (Tons ha 1 y 1) 28,000

Drought-tolerant, phytoremediation, sources of drugs

Extensive single species stands along the margins of lakes and streams and in wet open areas One of the fastest-growing plants, alcoholic and phenolic compounds Artichoke oil, feedstock for the first biorefinery and biodiesel Source of essential oils, phenolic compounds, medicinal compounds, fast growing Resistant to drought, used in dissolving pulp, durable yields, impressive bioenergy feedstock Biofuels, carbon mitigation potential, and fast growing

Potential characteristics Fast-growing, perennial, aquatic weed

1

1

97 GJ ha

92 TW h

1

1

2281 CH4 kg

1

233–245 GJ ha

138 GJ ha

41.85 GJ ha

Energy potential 124–127

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Nassi o Di Nasso et al. (2013) Saha and Eckelman (2015) Lord (2015)

Mehmood et al. (2017a) Francaviglia et al. (2016) Wadhwani et al. (2017)

Alhumade et al. (2019)

References Ruan et al. (2016)

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Perennial, native to sandy or rocky river shores, temperate

Virginia mallow (Sida hermaphrodita) Wolffia arrhiza

Smallest vascular plant on earth rich in water-loving species

Perennial, adaptive to versatile growth conditions, C4-pathway

Switchgrass (Panicum virgatum L.) 9–20

1–22

Ethanol, butanol, biogas production, thermal energy, phytoremediation, drought, and flooding tolerant Source of food and nectar for honeybee rearing, the potential for bioenergy production Wolffia arrhiza is a species of flowering plant

1

171 kJ ha

1

219.5 GJ ha

60 GJ ha

1

Ahmad et al. (2018)

Nassi o Di Nasso et al. (2015) Ţîţei (2017)

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Correlation of the Biomass Characteristics with the Biomass Valorization Techniques

Nature and biomass composition affects the down-streaming processing which offers various benefits along with the challenges. Valorization analysis of hemicellulose and cellulose shows that it has a high capacity to produce biochemicals, biofuels, paper, pulp, and animal feed. For example, to achieve the attractive target agro-industrial residues such as rice husk (Singh et al. 2014), pruning waste (Martín et al. 2010), sugarcane bagasse (de Araujo Guilherme et al. 2019), fruit peels (Gordon et al. 2011), coffee cut-stem (Aristizábal-Marulanda et al. 2020), and wood materials have been excessively employed for the production of biofuels (Poveda-Giraldo et al. 2021). Various biomasses including para grass (Asghar et al. 2021), Wolffia arrhiza (Ahmad et al. 2018), Calotropis procera (Ahmad et al. 2021), Chinese liquor-industry waste (Ye et al. 2018), and microalgae biomass (Agrawal et al. 2020; Goswami et al. 2021b, c, 2022; Mehariya et al. 2021; Shahid et al. 2019) have been produced using industrial wastewater and/or municipal wastewater. While the modification in the biochemical composition in response to wastewater cultivation showed positive impact on the biomass with respect to its usage as a bioenergy feedstock. The basic conversion processes include pyrolysis, gasification, torrefaction, and fermentation where either biochemical or thermal reactions are to progress (Akhtar et al. 2018). There are other less common, and more experimentally or exclusive thermal processes that may lead to cost-effectiveness, such as the hydrothermal upgrading process (Smith et al. 2018). The biomass with higher lignin content may be more suitable to produce biochar, while biomass with higher moisture and less lignin content may be more suitable for fermentation. Similarly, the biomass which contains some chemicals that have allelopathic impact can be processed using a two-stage biorefinery approach, where residual biomass can be used for bioenergy production through pyrolysis or fermentation after extracting the high-value biochemicals for applications such as medicine, preservatives, fragrance, cosmetics, and nutraceuticals.

1.5

Conclusion and Recommendations

Biorefinery is an environmentally friendly approach for the biotransformation of biomass with minimum waste and efficient resource recovery. The fate of biomass feedstock depends on its biochemical composition, source, cost-effectiveness, ease of transportation, and mechanism of processing. While cultivation conditions and seasons sometimes can be easily employed to modify the biochemical composition of the biomass for biorefinery applications. In the future, integration of the biorefinery with the urban wastewater treatment and phytoremediation systems could be cost-effective and could result in multifaceted opportunities. However,

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the impact of cultivation conditions, involvement of machine learning, modeling, and simulation-based studies should be conducted to refine the methodologies for achieving commercial robustness. Competing Interests All the authors declare that they have no competing interests.

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Martín JFG, Cuevas M, Bravo V, Sánchez S (2010) Ethanol production from olive prunings by autohydrolysis and fermentation with Candida tropicalis. Renew Energy 35(7):1602–1608 Mehariya S, Goswami RK, Verma P, Lavecchia R, Zuorro A (2021) Integrated approach for wastewater treatment and biofuel production in microalgae biorefineries. Energies 14(8):2282 Mehmood MA, Ibrahim M, Rashid U, Nawaz M, Ali S, Hussain A, Gull M (2017a) Biomass production for bioenergy using marginal lands. Sustain Prod Consump 9:3–21 Mehmood MA, Ye G, Luo H, Liu C, Malik S, Afzal I, Xu J, Ahmad MS (2017b) Pyrolysis and kinetic analyses of Camel grass (Cymbopogon schoenanthus) for bioenergy. Bioresour Technol 228:18–24 Nassi o Di Nasso N, Roncucci N, Bonari E (2013) Seasonal dynamics of aboveground and belowground biomass and nutrient accumulation and remobilization in giant reed (Arundo donax L.): a three-year study on marginal land. Bioenergy Res 6(2):725–736 Nassi o Di Nasso N, Lasorella M, Roncucci N, Bonari E (2015) Soil texture and crop management affect switchgrass (Panicum virgatum L.) productivity in the Mediterranean. Indus Crops Prod 65:21–26 Panwar N, Kaushik S, Kothari S (2011) Role of renewable energy sources in environmental protection: a review. Renew Sust Energ Rev 15(3):1513–1524 Płaza GA, Wandzich D (2016) Biorefineries—new green strategy for development of smart and innovative industry. Manage Syst Prod Engi 23(3):150–155 Poveda-Giraldo JA, Solarte-Toro JC, Alzate CAC (2021) The potential use of lignin as a platform product in biorefineries: a review. Renew Sust Energ Rev 138:110688 Ruan T, Zeng R, Yin XY, Zhang SX, Yang ZH (2016) Water hyacinth (Eichhornia crassipes) biomass as a biofuel feedstock by enzymatic hydrolysis. Bioresources 11(1):2372–2380 Saha M, Eckelman MJ (2015) Geospatial assessment of potential bioenergy crop production on urban marginal land. Appl Energy 159:540–547 Saini JK, Saini R, Tewari L (2015) Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments. 3 Biotech 5(4): 337–353 Sanna L, Uda M, Pascucci V (2016) Hazards in the coastal karst of Balai (NW Sardinia, Italy). EGU General Assembly Conference Abstracts pp. EPSC2016-17344 Shahid A, Ishfaq M, Ahmad MS, Malik S, Farooq M, Hui Z, Batawi AH, Shafi ME, Aloqbi AA, Gull M (2019) Bioenergy potential of the residual microalgal biomass produced in city wastewater assessed through pyrolysis, kinetics and thermodynamics study to design algal biorefinery. Bioresour Technol 289:121701 Singh A, Bajar S, Bishnoi NR (2014) Enzymatic hydrolysis of microwave alkali pretreated rice husk for ethanol production by Saccharomyces cerevisiae, Scheffersomyces stipitis and their co-culture. Fuel 116:699–702 Singh D, Goswami RK, Agrawal K, Chaturvedi V, Verma P (2022) Bio-inspired remediation of wastewater: a contemporary approach for environmental clean-up. Curr Res Green Sustain Chem:100261 Smith AM, Whittaker C, Shield I, Ross AB (2018) The potential for production of high quality bio-coal from early harvested Miscanthus by hydrothermal carbonisation. Fuel 220:546–557 Tey SY, Wong SS, Lam JA, Ong NQX, Foo DCY, Ng DKS (2021) Extended hierarchical decomposition approach for the synthesis of biorefinery processes. Chem Eng Res Des 166: 40–54 Ţîţei V (2017) The evaluation of biomass of the Sida hermaphrodita and Silphium perfoliatum for renewable energy in Moldova. Sci Pap Ser A Agron 60:534–540

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Chapter 2

Recalcitrance of Lignocellulosic Biomass and Pretreatment Technologies: A Comprehensive Insight Rohit Rai, Vikash Kumar, and Prodyut Dhar

Abstract Plant biomass is an excellent lignocellulosic source that can produce renewable and environment-friendly biofuels. However, the natural physiochemical structure of plant lignocellulose has strong recalcitrance and heterogeneity, which results in low yields of biofuels, limiting its effective valorization in biorefineries. This rigidity of lignocellulose presents economic and technical challenges in biomass conversion processes. Various pretreatment methods are used separately and in combination to resolve this. Pretreatment methods change the structure and chemical composition of the plant biomass, which makes it more accessible to the conversion systems for biofuel production. This chapter will discuss the physical and chemical basis of lignocellulose recalcitrance and the biomass components contributing to it. This chapter will also explain the role of pretreatment strategies in biorefineries and their influence on the structure and composition of lignocellulosic biomass. The fundamental understanding of biomass recalcitrance and the role of pretreatment methods can aid in the efficient utilization of lignocellulosic biomass in biorefineries and the development of future pretreatment methodologies. Keywords Lignocellulose · Recalcitrance · Pretreatment · Biorefinery

Abbreviations [EMIMAc] ASA BET

1-Ethyl-3-methylimidazolium acetate Accessible surface area Brunauer-Emmett-Teller

R. Rai · V. Kumar · P. Dhar (*) School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India e-mail: [email protected]; [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_2

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CSC CESA CSL DES DP G Glu GT H IPCC LCB LCC NMMO S Tg TSA UDP UTP X

2.1

Cellulose synthase complex Cellulose synthase A Cellulose synthase-like family Deep eutectic solvents Degree of polymerization Guaiacyl Glucopyranosyl Glycosyltransferases p-hydroxyphenyl Intergovernmental Panel on Climate Change Lignocellulosic biomass Lignin-carbohydrate complexes N-methylmorpholine N-oxide Syringyl Glass transition temperature Total surface area Uridine diphosphate Uridine triphosphate Xylopyranosyl

Introduction

The rapid industrialization and population growth this century have created a high demand for energy and materials, which is essential for the proper functioning of day-to-day life. They hold a huge economic significance and their scarcity can have a high impact on the stability of the global economy, industries, technological advancement, and diplomacy around the world. Conventional fuels and materials are petroleum-based, whose reserves are depleting rapidly in several nations. It also has huge drawbacks due to the environmental harms associated with it such as global warming, water pollution, and climate change (Foston and Ragauskas 2012). According to the Intergovernmental Panel on Climate Change (IPCC) report 2021 (IPCC report 9 Aug 2021), global warming is increasing rapidly and can cross 1.5
C of global warming in coming decades, if the quick, large-scale reduction in greenhouse gas emissions is not done. Another highlight of the report states that climate change is happening globally and affecting almost every region on the planet severely. The change in the water cycle, rainfall and flooding patterns, and sea-level rise in the recent decades is prominent proof of this. Most of the harmful environmental emissions originate from conventional petroleum-based sources. These limitations have incited the search for alternative fuel and material resources in recent decades that have the potential to replace conventional resources (Foston and Ragauskas 2012). In that context, alternative fuels and materials from plant biomass are becoming an upcoming choice as it is the most abundant biomass source

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in the world. It also has exciting features such as renewability, low cost, sustainability, and environment-friendliness that suggest its potential in becoming the leading source of fuels and materials in the coming future. However, utilization of plant biomass to obtain bioproducts is not as simple as it contains an inherent resistance to various types of treatments (chemical, physical, and biological) which is called biomass recalcitrance. The biorefinery approach has been utilized in recent years to solve this complication by using plant biomass to obtain products like biogas, biofuels, pulp and paper, building block chemicals, adhesives, and surfactants. Biorefineries are facilities that have a role in changing biomass into several valueadded products and are usually characterized by the feedstock used; for example, forest-based biorefineries, corn-based biorefineries, and algae-based biorefineries. Some challenges that are present in using the biorefinery approach are variation in biomass composition, production of fermentation inhibitors, and quality control. In addition, the effective performance of pretreatment and conversion strategies available during industrial scale-up are for a wide variety of biomass are the major hurdles (Agrawal et al. 2020; Goswami et al. 2021; Kamm 2014; Ng et al. 2017). This chapter discusses the structure, properties, and synthesis of various chemical components of plant biomass (lignocellulosic) and its recalcitrance concerning the contribution of various physical and chemical components present. It also highlights the role of pretreatment in biorefineries and the change in physical properties and chemical composition of lignocellulosic biomass on the application of various pretreatment processes.

2.2

Lignocellulosic Biomass (LCB)

The plant biomass is composed of lignocellulose that has lignin (5–30%), hemicellulose (20–35%), and cellulose (35–50%) as its components. The collective term for cellulose and hemicellulose is holocellulose which are polysaccharides with high molecular weight (325–490 kDa) (Bjurhager et al. 2012) whereas, lignin is made up of polymerized phenylpropanoic acids into a three-dimensional (3D) structure. Lignocellulose components are present in the form of a heteromatrix in which the individual components are linked extensively. The complex arrangement of cellulose, hemicellulose, and lignin in the cell wall is unique. Cellulose arranges itself in a fibrous crystalline structure and is present in the core of the lignocellulosic complex as presented in Fig. 2.1(a). Hemicellulose is present between both macro- and microfibrils of cellulose whereas lignin is present in the outer part of the complex and has a structural and protective role (Bajpai 2016). The other constituents present in the LCB are extractives (11.4%), pectin (10–35%), ash (8.52%), and proteins (10%) (Alvarez-Barreto et al. n.d.; Hernández-Beltrán et al. 2019). The ratio of lignocellulosic components (cellulose, hemicellulose, lignin) in plants varies across different species, and even in the same plant, it may vary depending on the stage of growth and age. The various forms of LCB present are herbaceous (lettuce, ferns), agricultural (rice, wheat), softwoods (balsa, spruce), and

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Fig. 2.1 (a) Schematic representation of lignocellulose complex containing cellulose, hemicellulose, and lignin (b) schematic representation of changes induced after pretreatment of lignocellulosic biomass

hardwoods (birch, hibiscus) (Bajpai 2016). The woody biomass of hardwoods and softwoods has a different lignocellulosic composition. Hardwoods have a higher amount of cellulose and hemicellulose with fewer amounts of lignin (18–25%) whereas softwood has a comparatively higher amount of lignin (25–35%). The lignin in softwood is mainly composed of coniferyl alcohol and is termed “guaiacyl lignin.” In contrast, hardwood lignin consists of coniferyl and sinapyl alcohols which is termed as “syringyl-guaiacyl lignin.” The ratio of these two lignin types of lignin mentioned varies in different lignin in a ratio of 4:1–1:2 (Demirbaş 2005; Rowell et al. 2005). A significant difference in non-woody (e.g., agricultural) and woody biomass (softwood and hardwood) is their chemical composition and physical properties. Woody biomass is structurally stronger, denser, and physically larger than the non-woody plants. Woody plants also have higher lignin contents than other groups. Generally, LCB biorefineries utilize agricultural biomass and their by-products because it is a cheap waste substrate that could be used in biorefineries for yielding high value-added products (Kumar et al. 2020; Kumar and Verma 2021; Zhu and Pan 2010). The diversity in the lignocellulosic components in biomass and

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their interactions significantly affects the selection and optimization of appropriate pretreatment methods which in turn affects the cost and economical aspects of the bioproducts made out of it (Demirbaş 2005; McMillan 1994).

2.2.1

Chemical Interactions in Lignocellulosic Biomass

The components of lignocellulose are bound to each other with several linkages which helps in designing the complex structure of LCB. There are five types of bonds found in the lignocellulose structure that are (i) hydrogen (–H) bonds, (ii) carbon-carbon (C–C) bonds, (iii) ester bonds, (iv) ether bonds, and (v) acetal bonds. These linkages have a major role in forming two types of bonding interactions, i.e., bonding within a component (intramolecular bonding) and bonding between the components (intermolecular bonding). The intermolecular bonds formed during lignin biosynthesis between the lignin and the carbohydrates (cellulose and hemicellulose) and the combined structure of lignin and carbohydrates are called lignin-carbohydrate complexes (LCC). During its formation, lignin substitutes most of the water in the cell wall and results in making a tough solid matrix by forming covalent interactions with the carbohydrates present. To obtain value-added products from LCB, its components need to be separated so that the individual components of biomass become more accessible to treatments. The LCC makes biomass processing in biorefineries challenging as it inhibits the fractionation and isolation of individual components (Harmsen et al. 2010; Tarasov et al. 2018). The type of linkages that are present in LCC are (a) benzyl ether bonds, (b) glycosidic or phenyl glycosidic bonds, (c) benzyl ester bonds, and (d) acetal or hemiacetal bonds. Benzyl ether bonds α-carbon of mannosyl or glucosyl residues of the carbohydrates with the arylpropane unit in lignin whereas glycosidic bonds have a function to link side-chain hydroxyl moieties of the lignin with the carbohydrates. In the case of phenyl glycoside, the lignin which is linked by glycosidic bonds has a phenolic hydroxyl group. Benzyl ester bonds link carbohydrates (mostly xylan) with lignin by utilizing uronic acid of sugar and hydroxyl (OH) group (at α or γ carbon) of lignin. Acetal bonds are formed by the hydroxyl group of carbohydrates and the carbonyl group of phenylpropane of lignin molecules. These linkages in LCC almost combine all the lignin present in the wood with cellulose and hemicellulose and are important for valorization aspects (Tarasov et al. 2018; Zhao et al. 2020). The intramolecular bonding between individual components of lignocellulose will be discussed later in the chapter.

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Components of LCB: Cellulose, Hemicellulose, and Lignin Derivatives

2.3.1

Cellulose: Structure, Synthesis, and Properties

Cellulose holds its position as the most abundant natural polymer worldwide and has an annual global production of approximately 1.5  1012 tons (Klemm et al. 2005). Rapid research in the previous decades in the field of cellulose has enabled its utilization at a large scale in industries. The properties of cellulose such as biodegradability and renewability are attractive features and its properties can be further modified by physical and chemical modifications. This broadens its potential applications in industries such as textiles, coating materials, optical films, and functional materials (Heinze and Liebert 2012; Mohanty et al. 2000). Cellulose is present abundantly in plant species such as Kapok (Ceiba pentandra) (70–75%), Jute (Corchorus) (60–65%), Hemp (Cannabis sativa ssp. sativa) (70–75%), and Bamboo (Bambusa vulgaris) (40–55%). Cotton (Gossypium) plants are the most common cellulose source and contains a high amount of cellulose (~90 wt%) (Hon 2017). The structure and properties of cellulose influences its utilization for industrial applications. Since cellulose is the core component of the lignocellulose complex, its structural understanding and property evaluation is important to develop and select appropriate pretreatment techniques for the lignocellulose utilization in biorefineries.

2.3.1.1

Cellulose Structure

Cellulose structure is composed of D-glucopyranose moieties in a 4C1 chair type configuration which is the minimum energy conformation. The individual glucose rings are joined with β-(1,4) glycosidic linkage in which the cellulose chain turns alternatively by 180 degrees as presented in Fig. 2.2(a) (Krässig 1993; Rao et al. 1967). The hydroxyl groups are found at the C2, C3, and C6 position on the glucopyranose ring in the cellulose chain which can participate in various chemical reactions as shown in Fig. 2.2(b). The ends of the cellulose chain are different from each other. One end of the chain is non-reducing in which the anomeric carbon participates in the formation of glycosidic bonds and contains a pendant hydroxyl group whereas the other end is reducing in which the D-glucopyranose unit contains an aldehyde group (Krässig 1993). The hydrogen bonding in and between the cellulose chains and its pattern plays a significant part in the determination of cellulose chain properties such as reactivity, solubility, and crystallinity (Kondo 1997). Both intramolecular and intermolecular hydrogen bonds are present in cellulose chains that are formed by the interaction of hydroxyl groups and glucose ring oxygen atoms, and glycosidic linkages in the cellulose chain. Intramolecular hydrogen bonds are the reason for cellulose chains rigidity and high viscosity of the solutions formed. On the other hand, the intermolecular hydrogen bonds provide strong linkage among chains of cellulose. The hydrogen bonding results in the

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Fig. 2.2 (a) Structure of cellulose chain containing glucopyranose units linked with β-(1,4) bonds (b) participation of hydroxyl groups of at C2, C3, and C6 position of glucose in cellulose chain into various chemical reactions

creation of three-dimensional crystal structures of cellulose chains (Gardner and Blackwell 1974; Krässig 1993). The six different crystalline cellulose types are cellulose I, II, IIII, IIIII, IVI, IVII, also called cellulose polymorphs (Gardiner and Sarko 1985; Wada et al. 2001; Zugenmaier 2001). In natural systems, only cellulose I polymorph is present and the other polymorphic forms are derived from it by chemical treatment. The native “cellulose I” is further categorized into two different allomorphic forms, namely Iα and Iβ and their proportions differ from species to species in plants. In LCB, the Iβ form is present abundantly. Cellulose Iα has a triclinic P1 unit cell, and each unit cell has a single chain. The dimensions and angles of the unit cell of cellulose Iα are a ¼ 6.72 Å, b ¼ 5.96 Å, c ¼ 10.40 Å, α ¼ 118.08o, β ¼ 114.80
, and γ ¼ 80.38
(Poma et al. 2016). On the other hand (Moon et al. 2011), Iβ allomorph of cellulose I have a monoclinic P21 unit cell, and each unit cell has two cellulose chains. The dimensions and angles of the unit cell are a ¼ 7.78 Å, b ¼ 8.20 Å, c ¼ 10.38 Å, α ¼ β ¼ 90
, and γ ¼ 95.5
(Poma et al. 2016). The average degree of polymerization (DP) of cellulose chains in plants is between 1000 and 30,000 (Gardner and Blackwell 1974; Krässig 1993; Poma et al. 2016).

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Fig. 2.3 Schematic representation of different layers presents in plant cell wall and arrangement of cellulose fibers in these layers

In crystalline forms of cellulose, the pattern of chain alignment is close and ordered which is linked side by side with hydrogen bonds. Amorphous cellulose is shortly ordered and linked by isotropic intermolecular hydrogen bonding. The amorphous regions of cellulose chains; however, help in forming turns between adjacent chains. The amorphous regions also display random orientations and low density (de Souza Lima and Borsali 2004). The cellulose chains in the cell wall are present as fibrils that have their diameter in the nanometer (nm) range. These fibers arrange themselves in bundles to form microfibrils and microfibrils during which the magnitude of dimensions increase from nm to micrometers (μm) (Krässig 1993). During growth, plant cells first form the primary cell wall which is thinner than secondary and tertiary cell walls. The primary and tertiary walls have a disordered arrangement of cellulose nanofibrils whereas cellulose microfibrils are arranged parallelly in secondary wall sublayers as presented in Fig. 2.3 (Rytioja et al. 2014). The morphologies and dimensions of cellulose in cell walls differ from species to species. For example, spruce cellulose fibers are untwisted and cotton fibers are twisted whereas straight and round fibers are found in bast plants. However, these microfibrils have the same internal structure across the various plant species (Ioelovich 2008).

2.3.1.2

Chemical Properties of Cellulose

Pure cellulose is insoluble in the water because of its crystalline structure, hydrogen bonding, and high molecular weight. Due to this, special procedures and mediums are used to dissolve cellulose. The solvents that are used for the dissolution of cellulose are segregated into two categories, namely (i) derivatizing and (ii) non-derivatizing solutions. Derivatizing solutions (e.g., CF3COOH, HCOOH) form covalent linkages of low stability with the polymer whereas non-derivatizing solvents (e.g., dimethylsulfoxide, ammonia) only interact physically with the polymers. An important process used for cellulose dissolution at the industrial level is with the mixture of copper hydroxide and aqueous ammonia (Schweizer’s reagent), followed by precipitation in dilute sulfuric acid which is the part of fiber production in the viscose process. An alternate strategy for dissolution of cellulose is by dissolving it physically with N-methylmorpholine-N-oxide monohydrate

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(derivatizing) which is used in the Lyocell process (Heinze and Liebert 2001). Most of the solvents that are used for cellulose dissolution are developed and processed at the laboratory scale. Some of the semi-industrial scale solvents utilize the applications of ionic liquids such as salts of 1-alkyl-3-methylimidazolium [(1-Ethyl-3methylimidazolium acetate) (EMIMAc)] (Ebner et al. 2008; Liebert 2008; Swatloski et al. 2002). Ionic liquid for cellulose dissolution came into existence in the year 2002 and are proved to be a good solvent for cellulose because the anions in the ionic liquids are always accompanied by equal numbers of cations to maintain the electroneutrality. Different ionic liquid display difference in solubility and spinning characteristics for cellulose. For example, EMIMAc dissolved cellulose display a poor spinnability whereas N-methylmorpholine N-oxide (NMMO) cellulose solutions shows high draw ratios during spinning (Hauru et al. 2016). Even before the nature of cellulose was understood completely, glucan cellulose was utilized as chemical modification precursor for cellulose. The hydroxyl groups of the cellulose participate in the chemical reactions and several industrially important compounds are obtained from cellulose modifications. For example, cellulose in the partially nitrated form is sold commercially with the trade name of “Celluloid” which acts like plastic. Another important chemically modified cellulose product is cellulose acetate which is highly utilized in industries and research. The chemical modifications of cellulose include acylation, sulfation, amino modification, carbonylation, silylation, and oxidation (Gerhartz 1988; Habibi 2014). However, the mechanisms of these chemical modifications are beyond the scope of this chapter and are mentioned to give an idea about varieties of chemical modifications of cellulose available.

2.3.1.3

Mechanism of Cellulose Synthesis in Plants

The synthesis of cellulose in plants is performed by cellulose synthase (CSC) enzymes. It is present as a rosette-like complex in the cell membrane of plants. The catalytic part of the CSCs is called CESA protein (cellulose synthase A) (Chen et al. 2018). CESA subunits arrange themselves to form a rosette-type structure which is composed of 36 CESA subunits. These 36 CESA are organized into six subunits, and each subunit contains six CESA units. CESA is present as a complex in the cell membrane that can synthesize cellulose (Endler and Persson 2011; Turner and Kumar 2018). The CESA genes are responsible for the synthesis of cellulose in plants. Some CESA genes are necessary for primary cell wall formation (CESA1, CESA3, CESA6), whereas some are needed for the construction of secondary cell wall (CESA4, CESA7, CESA8). During cellulose synthesis in plants, glucose subunits are polymerized by CESA to form cellulose. The glucose subunits are utilized as uridine diphosphate glucose (UDP-glucose). The enzyme UDP-glucose phosphorylase forms UDP-glucose by utilizing uridine triphosphate (UTP) and glucose1-phosphate (Guerriero et al. 2010). Sucrose synthase (Susy) enzyme has also been assumed to connect cellulose synthesis by converting sucrose and UTP into UDP-glucose. During the initialization of cellulose synthesis, the enzyme has two

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UDP-glucose units in the substrate-binding site, and after that, polymerization begins. During polymerization, glucose subunits are introduced to the non-reducing end of the chain, and the chain is released in the extracellular part simultaneously. This exported cellulose chain then assembles and crystallizes to form cellulose microfibrils (Brown Jr and Saxena 2000). The system for plant cellulose synthesis is based on the mechanism of Arabidopsis thaliana, which is used as a model for plant species (McFarlane et al. 2014).

2.3.2

Hemicellulose: Structure, Properties, and Synthesis

Hemicellulose is the most abundant polysaccharide after cellulose in the plant’s system. They are also the most abundant heteropolysaccharides found in nature. The hemicellulose family is diverse in context to their origin and relative proportion to cellulose in the plant cell wall. On a molecular level, hemicellulose displays a high variation in the degree of branching and molecular weight. However, their DP is comparatively less than cellulose (Timell 1967). In-plant species, the composition of hemicellulose differs in terms of the types of sugar and their respective amounts. Even in the same plant, several forms and amounts of hemicellulose may exist in different macroscopic structures (root, core, branches). The difference in hemicellulose composition and the amount is also observed in different cell wall layers and within the wood structure (heartwood, latewood, and earlywood). Hemicellulose composition in the hardwoods is quite similar in species such as birch and aspen and is composed mainly of xylan (high amount) and mannan (low amount). In the case of softwood, the major monomer is glucomannan, and arabinogalactans and xylans are present in the smaller amounts. In recent decades, there has been a huge development towards complete characterization of the hemicellulose, providing a comprehensive understanding of hemicellulose structure which was a great challenge in the last century (Albertsson et al. 2011).

2.3.2.1

Structure of Hemicellulose

The hemicellulose is a complex heteropolysaccharide composed of a single or combination of pentoses and hexoses. The pentoses involve monosaccharides such as xylose and arabinose whereas hexoses include D-galactose, D-glucose, D-mannose, D-glucuronic acid, etc. (Aspinall 1959; Aspinall 1962). Fucose and rhamnose sugars are also found in some hemicelluloses. The composition of sugars varies in plant species based on the tissue of origin and geographical location. The representative structure of hemicellulose is considered as β-(1,4) linked xylopyranose subunits that are present in various plant parts including stem, root, and leaves. The complexity of the hemicellulose structure is enhanced further by branching in the form of side groups at the C2 and C3 positions of the sugar. Examples of these side groups are

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Fig. 2.4 (a) Structure of different types of hemicellulose present in plant cell wall (i) xylan, (ii) mannan, (iii) xyloglucan, (iv) β-glucans (b) structure of lignin monomers (i) p-coumaryl alcohol, (ii) coniferyl alcohol, (iii) sinapyl alcohol

glucuronic acid, rhamnose, arabinofuranose, and phenolic acids. The phenolics like coumaric acids have antimicrobial, antioxidant, and anti-inflammatory properties (Adams and Castagne 1951; Aspinall 1962; Ghali et al. 1974). Due to the different kinds of sugars involved in the hemicellulose structure, they are categorized into four types that are (i) xylans (ii) mannoglycans or mannans, (iii) xyloglucans, (iv) β-glucans (Scheller and Ulvskov 2010) as shown in Fig. 2.4(a). The first group, xylans are the most common and abundant form of hemicellulose. It is composed of xylopyranosyl subunits linked with β-(1,4) bonds. It is substituted by branches containing arabinosyl, glucuronosyl, and acetyl residues based on the extraction method and biomass source (Hauru et al. 2016). The xylan found in straw has L-arabinofuranose in its side chains. Some plants also contain the acidic xylans in which the acidic group is provided by the 4-O-methyl-D-glucoronic acid. The complex forms of xylans contain xylopyranose, galactopyranose, furanose, and uronic acids as their side groups. The xylans that contain simple chains of

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xylopyranosyl subunits linked with β-(1,3) and β-(1,4) or both the bonding are called homoxylans whereas the xylans that contain a large variety of sugars are called heteroxylans (Ebringerová and Heinze 2000; Scheller and Ulvskov 2010; Teleman et al. 2000). Mannans are the second group of hemicellulose and are abundantly present in the softwood hemicellulose. The mannans are of four types: (a) mannan, (b) galactomannans, (c) glucomannan, (d) (galacto)glucomannans. Mannans are composed of mannose sugar linked with β-(1,4) bonding whereas the galactomannans are composed of D-mannopyranosyl subunits with β-(1,4) bonding. At C6 position, the D-galactopyranosyl with short branching are linked to mannan units in galactomannans. The glucomannans contain a D-mannopyranosyl subunit liked with β-(1,4) linkages. Some glucomannans have D-galactopyranosyl components as their side groups and therefore and are called (galacto)glucomannans. In softwoods, the mannans are usually acetylated at C2 and C3 positions. Mannans are utilized commercially in food industries as thickeners and emulsifiers (González 1978; Scheller and Ulvskov 2010). The third group, xyloglucans are formed of D-glucopyranose subunits linked with β-(1,4) linkages and are present within primary cell walls of angiosperms, grasses, and fruit plants (Fry 1989; Yapo and Koffi 2008). The xylopyranosyl (X) subunits combine with glucopyranosyl (Glu) at C6 position with α-linkages, based on which xyloglucans are further categorized into two groups: (a) XXXGlu group and (b) XXGluGlu group. XXXGlu groups have four glucopyranosyl repetitive subunits, of which three contain xylopyranosyl units. In the case of XXGluGlu, two of the four repetitive glucopyranosyl subunits are xylosylated followed by two non-substituted units (Cui and Wang 2009). The last group of hemicellulose is β-glucans which are linear hemicelluloses containing β-D-glycopyranosyl subunits similar to the structure of cellulose. The main difference between cellulose and β -glucans is that the cellulose only contains β-(1,4) linkages whereas β-glucans contains both β-(1,3) and β-(1,4) bonds. Two types of building blocks are found in the β-glucans: (a) cellotriosyl and (b) cellotetraosyl. The cellotrisyl units are composed of two continuous β-(1,4) bonds and a β-(1,3) linkage is present in between them. On the other hand, the cellotetraosyl has three repetitive β-(1,4) bonds followed by a β-(1,3) bond. The relative abundance of cellotriosyl and cellotetraosyl varies from one plant to another and depends on the growing environment. For example, barley is found to be rich in β-glucans whereas wheat hemicellulose is majorly composed of arabinoxylan (Cui and Wang 2009; Izydorczyk et al. 1998). The structural aspects of hemicellulose are crucial to understanding as it has a significant effect on the properties of this polysaccharide which eventually affects the properties of LCB.

2.3.2.2

Properties of Hemicellulose

The physical, chemical, and functional properties of hemicellulose depend on the DP, molecular mass, macroscopic structure, and branching (Belgacem and Gandini

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2011). The hemicellulose is different from cellulose as it contains both pentose and hexose sugars and displays a lower DP. Hemicellulose is non-crystalline and is more accessible to utilization compared to cellulose (Dumitriu 2004). However, like cellulose, hemicellulose is also insoluble in water and there are several methods used for their solubilization and recovery. Arabinoxylan or β-(1,4) xylan (unsubstituted) forms insoluble complexes by aggregation which is stabilized with intermolecular hydrogen bonding. It forms a left-handed helix having three folds which appear as an extended twisted ribbon-like structure in solid-state. The formation of aggregated complexes could be the most probable reason for their low solubility in water (Belgacem and Gandini 2011). Xylans from hardwoods can also be solubilized in alkaline solutions which is a high yield method. The products obtained out of this solubilization are similar to native hemicellulose, except all the acetyl side chains, p-coumaryl, and feruloyl moieties are saponified (Puls 1997). Similar to cellulose, hemicellulose could also be extracted using ionic liquids which is an upcoming strategy for dissolving individual components of the lignocellulosic components. Ionic liquids such as 1-butyl-3-methylimidazolium chloride (BmimCl), and 1-butyl-3-methylimidazolium bromide (BmimBr) can be used to dissolve hemicellulose (Yuan et al. 2019). In hardwood, xylans are present abundantly whereas in softwood mannans are most common. Both these groups contain hydroxyl groups with them which allows them to participate in several chemical reactions with multiple chemical agents. These reactions are performed for the enhancement of the properties such as hydrophobicity, thermal stability, and water resistance. The chemical reactions and modifications such as acetylation, esterification, etherification, fluorination, benzylation, and cross-linking could be done with hemicellulose which widens its scope of application as a polymer. For example, hemicellulose has the ability to react with acids, chlorides, and acid anhydrides. The substitution of the hydroxyl group in this reaction will affect the properties of hemicellulose such as thermoplastic behavior and waterproofing. The degree of substitution with reaction with acyl chlorides is affected by reaction conditions such as reaction temperature, time, and the molar ratio of the substrate. Another example of hemicellulose participating in reaction would be esterification which is generally performed with a fatty acid chloride. This modification has shown an increase in the water-resistance properties due to the presence of long-chain fatty acids on the surface and can be used for coating applications (Hu et al. 2020).

2.3.2.3

Hemicellulose Synthesis in Plants

From the previous sections, we know that hemicelluloses are composed of a variety of sugars. The biosynthesis of these individual sugar constituents varies depending on the type of sugars which is being synthesized. In the hemicellulose, the mannans, xylans, xyloglucans, and mixed-linkage glucan biosynthesis are generally considered. Similar to all the polysaccharides that are present in the plant cell wall, hemicelluloses are also synthesized by the activated nucleotide sugars such as

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guanosine diphosphate (GDP)-glucose, uridine diphosphate (UDP)-galactose, and GDP-mannose. The enzyme cellulose synthase-like family (CSL) are glycosyltransferases that majorly participate in hemicellulose synthesis. The proteins from this family such as CSLA, CSLC, and CSLH are found in the Golgi of the plant cell and have a huge role in the synthesis of mannan, xyloglucan, and mixed-linkage glucans, respectively. They synthesize the hemicellulose components in Golgi and export them to the cytoplasmic side for deposition into the cell wall. The enzyme βmannan synthase (ManS) is an example of a CSL enzyme that is responsible for the heteromannan synthesis and uses GDP-mannose as a substrate (Pauly et al. 2013). Although CSL proteins have a huge role in hemicellulose synthesis in plants. For backbone synthesis in xylan, these proteins do not participate. The proteins belonging to glycosyltransferases (GT) GT43 and GT47 (present in Golgi) are responsible for xylan backbone synthesis. These proteins are type II membrane proteins with a single membrane anchor at N-terminal which is different from CSL family multiple transmembrane segments. The processes involved in hemicellulose biosynthesis are backbone formation, modifications by hydrolases, acetylation, reducing end modification, and formation of side chains of the participating constituents (Scheller and Ulvskov 2010).

2.3.3

Lignin: Structure, Properties, and Synthesis

Lignin comes after cellulose and hemicellulose polysaccharides in terms of polymer abundance in nature. Lignin is a complex aromatic phenylpropanoid polymer that is associated with the cellulose and hemicellulose in the plant cell wall (Boerjan et al. 2003). It helps in cementing these polysaccharides in the cell wall chemically and physically. Lignin has a structural and protective role in the plants which makes them rigid and inaccessible to biotic and abiotic stress conditions (Kirk 2018). It also aids in plant growth and development by enhancing water conduction through xylem vessels. In the industrial valorization of the LCB, lignin has been always considered a non-desired product such as Klason lignin. Klason lignin is an insoluble residue that is obtained after the acid hydrolysis of plant biomass. However, the lignin wasted in the biorefineries can be utilized which will strengthen the economic value of lignocellulosic biomass-based industries. Lignin has been utilized as emulsifying agents, binding agents, reinforcing materials, and composites. The recent research in the direction of lignin extraction, depolymerization, and subsequent valorization has shown excellent results which suggests that it contains a huge potential from an industrial and commercial perspective and is yet to be unleashed (Ponnusamy et al. 2019; Whetten and Sederoff 1995).

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Structure of Lignin

The phenylpropane units that are involved in the lignin formation are called monolignols. The three types of monolignols that constitute lignin are (i) phydroxyphenyl (H ) (ii) guaiacyl (G), and (iii) syringyl (S) structures. These monolignols combine through radical-based polymerization to produce a threedimensional lignin structure. The building blocks of lignin are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol as presented in Fig. 2.4(b). Lignin is a heterogeneous material and does not have a regular structure like cellulose and the exact chemical structure of lignin is not completely defined (Hatakeyama and Hatakeyama 2009). The monolignols in lignin are bonded with carbon-carbon (C-C) and ether linkages. Trifunctionally linked adjacent units present branching sites that generate the network structure of lignin. The types of linkages that are present between the monolignols in the lignin are aryl glycerol-β–aryl ether (β–O– 4), non-ring benzyl-O-aryl ether (α–O–4), coumaran (β–5), 1,2-diaryl propane (β– 1), diaryl ether (4–O–5), and β–β (Lebo Jr et al. 2002). In gymnosperms, the lignin majorly consists of the G subunits with a small number of H units whereas in angiosperms dicot lignin majorly consists of G and S units. Softwood contains a high amount of H units which are abundant in grasses. The variation in lignin composition not only varies with taxa, species, growth conditions, and environment but also differs from cell to cell. It can be dissimilar even at the level of cell wall layers which was revealed by infrared, Raman, and ultraviolet spectroscopy. The concentration of lignin in the plant cell is uniform in the secondary cell wall; however, a high rise in the concentration of lignin is seen at the primary cell wall area and middle lamella periphery. In typical wood cells, the pattern of lignin distribution is like this with a high amount in the interfiber area and low concentration in the bulk of the cell wall. In the primary cell wall and middle lamella, the lignin is present as spherical entities, however, in the secondary cell wall, lamellae formation occurs that is based on microfibril orientation. In the lignocellulose, lignin is the most complex and least characterized component. Comprehensive studies on the structure and distribution of lignin in plant cell walls can potentially unravel the new findings which will further improve the understanding of this complex polymer (Boerjan et al. 2003; Vanholme et al. 2010).

2.3.3.2

Properties of Lignin

The general properties of native lignin are its high thermal stability, high carbon content, biodegradability, stiffness, and antioxidant activity. Lignin in the wood is present in an amorphous form in the cell wall and does not present an absorption peak in the visible light spectrum (Huang et al. 2019; Melro et al. 2018). The color in lignin derived from heartwood is a result of impurities such as flavonoids and tannins. Kraft and sulfite pulps are also brownish because of the presence of chromophoric moieties. Lignin shows UV absorption range which is the reason for

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the apparent color of lignin (Huang et al. 2019). Using techniques such as light scattering and vapor pressure osmometry, the milled softwood lignin molecular weight is calculated as 20 kDa. However, more variation to this molecular weight can happen depending on the method of estimation and wood source. For the solubility properties of lignin, like cellulose and hemicellulose, lignin is also an insoluble polymer. The lignin is soluble in solvents such as acetone, pyridine, and dioxane (Lebo Jr et al. 2002). The organic solvents are traditionally used for lignin extraction (e.g., methanol, acetic acid); however, it displays low solubility. An alternative to the organic-based solvents could be the use of ionic liquids that have demonstrated intriguing properties such as non-toxicity and high thermal stability. These solvents are also called “green solvents.” Some solvents that have been utilized for lignin dissolution are s 1,3-dimethylimidazolium methylsulfate, [Mmim][MeSO4] and 1-butyl-3-methylimidazolium methylsulfate, [Bmim] [MeSO4]. Another category of “green solvents” includes deep eutectic solvents (DES) which are formed by mixing a hydrogen bond donor and a hydrogen bond acceptor. Some of the DES that can be used for lignin dissolution is choline chloride and formic acid (2:1) and propionic acid and urea (1:2) (Melro et al. 2018). Lignin acts as a thermoplastic polymer and has a range of glass transition temperatures (Tg) which depends on the isolation method, heat treatment, and sorbed water. For example, the Tg of dioxane lignin was found to be 440 K. The process of delignification is enhanced by the thermal softening of lignin in chemical pulping. It also improves the strength of bonds of fibers in paper-making process (Atack and Mi 1978; Hatakeyama 1972). Lignin can participate in a variety of chemical reactions such as reduction, oxidation, and hydrolysis. It is also susceptible to enzymes. The hydroxyl, carbonyl, and benzylic hydroxyl groups in the phenolic structure of lignin enable it to participate in chemical reactions. The abundance of these functional groups varies depending on the wood species, morphological location, and isolation methods (Andersson and Samuelson 1985; Schuerch 1975). These chemical reactions result in reactive intermediates formation such as phenoxy anion and phenoxy radical. These radicals are significant in processes such as lignin isolation, enzymatic degradation, and light-induced discoloration. Lignin also demonstrates electrophilic substitution which aids in its modification process. The reactions that generally participate in lignin modification are nitration, ozonation, chlorination, etc. A clear characterization of the lignin properties is an important step towards lignocellulose valorization in biorefineries. The process involved in the treatment and extraction of individual components in lignocellulose can be strategized efficiently for the complete characterization of lignin properties (Melro et al. 2018).

2.3.3.3

Lignin Biosynthesis in Plants

Lignin biosynthesis in the plant systems begins with the formation of monolignols that are synthesized by the general phenylpropanoid pathway. In this, phenylalanine acts as a common precursor of monolignols throughout the plant systems. However,

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tyrosine also acts as a precursor in grasses. The phenylalanine first deaminates to form cinnamic acid. This reaction is performed by the enzyme ammonia-lyase which is one of the most studied enzymes in plant systems. The cinnamic acid obtained from this is then hydroxylated to p-coumaric acid by enzyme cinnamate 4-hydroxylase which is a monooxygenase. After the synthesis of p-coumaric acid, the lignin biosynthesis divides into two branches (Whetten and Sederoff 1995). The first branch sequentially forms caffeic acid, ferulic acid, and 5-hydroxyferulic acid that participates in the formation of G and S monolignols along with the intermediates from the other branch. The synthesis of caffeic acid by p-coumaric acid occurs through hydroxylation with the enzyme coumarate 3-hydroxylase. The second branch progresses through the conversion of p-coumaric acid to p-coumaroyl-CoA by enzyme coumaroyl-coenzyme A 3-hydroxylase. After the formation of pcoumaroyl-CoA, this pathway branches further in which one leads to the formation of caffeoyl CoA and feruloyl CoA whereas the other leads to the formation of pcoumaroyl alcohol. The p-coumaroyl alcohol participates in the formation of H monolignols whereas caffeoyl CoA and feruloyl CoA help in the formation of G and S monolignols (Mamedes-Rodrigues et al. 2018). Figure 2.5 shows a schematic representation of p-hydroxyphenyl (H ), guaiacyl (G), and syringyl (S) subunits biosynthesis pathway in plant cell along with the enzymes involved in each step. After the formation of monolignols, these are exported to the cell wall, where their oxidation and polymerization occurs. The accumulation of monolignol 4-O-β-D-glucosides in the cambial tissues in both gymnosperms and angiosperms is the basis of the hypothesis that these are the transport or storage forms of monolignols (Steeves et al. 2001). The completion of monolignols transfer to the cell wall is followed by the process of dehydrogenation and polymerization. The process of dehydrogenation helps in the formation of monolignol radicals and is performed individually and in combination by the enzymes (Kumar and Verma 2020a, b; Bhardwaj et al. 2021) such as laccases (Agrawal et al. 2019; Agrawal and Verma 2020), peroxidases, coniferyl alcohol oxidase, and polyphenol oxidases (Christensen et al. 2000; Onnerud et al. 2002). The formation of monolignol radicals then proceeds into the polymerization process by the radical coupling process. The radicals because of their electron delocalization are relatively stable and present in a coupled form. The extension of three-dimensional lignin polymer occurs by the cross-coupling reaction. However, these reactions quench the radicals; therefore, two coupling molecules require new radicals for every extension. These radicals in the synthesizing lignin polymer are produced with the transfer of radicals from monolignols or their intermediates. For example, in the sinapyl alcohol coupling reactions, p-coumarates function as intermediates. Due to their rapid oxidation by peroxidase, they can supply the radicals to sinapyl alcohol which is comparatively stable radical (Hatfield et al. 1997; Takahama and Oniki 1994). Oxidation of pcoumarates, therefore, occurs only when all sinapyl alcohols are polymerized. The radical coupling process completes the process of lignification in the cell walls of plants. The lignin is an intriguing component of LCB because of its aromatic nature and complexity. It also has a very significant role in providing LCB a resistant nature

Fig. 2.5 Biosynthetic pathway of p-hydroxyphenyl (H ), guaiacyl (G), and syringyl (S) subunits of lignin in plants cell

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towards the environmental agents (Boerjan et al. 2003). In the further sections, this resistance or recalcitrance of LCB will be discussed in detailed and the contribution of individual components will also be highlighted.

2.4

Recalcitrance of Lignocellulose Biomass (LCB)

Since plants are present in an environment where they interact with a large number of biotic and abiotic agents, they have been evolved to form a complex structure. This structure is difficult to deconstruct by the pathogens and protects the plants. This resistance of plant biomass is called biomass recalcitrance (Himmel et al. 2007; Ritter 2008). Due to this, the processing of LCB in refineries is difficult as it restricts the depolymerization of its components to their respective monomers. The recalcitrance of LCB possesses difficulty in bioproducts production. The components of the plant cell wall that were discussed previously have a huge contribution to the recalcitrance of LCB. LCB is composed of lignin, glycosylated proteins, and cross-linked networks of polysaccharides (Ritter 2008). The main factors that contribute to the lignocellulose recalcitrance are (i) physical properties of the plant cell (ii) chemical composition (iii) and their mixed interactions as displayed in Fig. 2.6 (Zhao et al. 2012a).

2.4.1

Effect of Physical Properties of LCB on Recalcitrance

2.4.1.1

Cellulose Crystallinity

The cellulose in LCB is present as crystalline and amorphous forms. The crystalline portion of cellulose exists in the form of microfibrils and dozens of these microfibrils combine to produce a paracrystalline assembly. This structure has a high number of hydrogen bonds and is more resistant to degradation by enzymes when compared to amorphous portions of cellulose (Laureano-Perez et al. 2005). The crystallinity of cellulose reduces with the enzymatic digestion of the cellulose. Since cellulose

Fig. 2.6 Schematic representation of physical properties and chemical composition-based factors contributing to lignocellulose recalcitrance

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crystallinity differs from species to species; therefore, the recalcitrance present by LCB from various biomass sources could be a result of it (Zhao et al. 2012a). However, the conversion of native cellulose to its polymorphic forms can reduce the extent of hydrogen bonding, making it more suitable for digestion (Chundawat et al. 2011). The amorphous portion of cellulose is more suitable for hydrolysis as it is more disordered and has lesser hydrogen bonds than crystalline cellulose (Zhang and Lynd 2004). The quantitative parameter to determine the cellulose crystallinity is called Crystallinity Index (C.I %) which is obtained using X-ray diffraction (Zhao et al. 2008). In LCB, hemicellulose and lignin are present that can interfere with the crystallinity index results of the cellulose and can result in an inaccurate result. To resolve this, biomass crystallinity is measured using methods such as infrared (IR) spectroscopy, Raman spectroscopy, and solid-state 13C NMR (Park et al. 2010). Although crystallinity is considered an important factor towards LCB recalcitrance, some studies also show that cellulose hydrolysis does not depend on the crystallinity of the cellulose in LCB. A possible reason for this could be that the use of pure cellulose for determining the effect of crystallinity on cellulose digestion does not give an idea about the heterogenous biomass (Chandra et al. 2007). The other structural properties that affect the LCB recalcitrance are DP of cellulose, pore volume, particle size, and surface area (Zhao et al. 2012a).

2.4.1.2

Degree of Polymerization (DP)

The number of monomers per unit of a polymer chain is called the degree of polymerization (DP). DP of cellulose defines the number of glucose molecules per unit of the cellulose chain. The process of enzymatic hydrolysis is depolymerization of the cellulose chains by an individual or synergistic action of enzymes such as endo-1-4-β-glucanase and β-glucosidase. It has been observed that hydrolysis of cellulose with shorter chains is much easier than longer chains (Duff and Murray 1996; Sánchez 2009). Similar to crystallinity, DP is also not an independent factor and affects the overall properties of the LCB. For example, the beating of LCB reduces the fiber DP but increases the swelling behavior. The studies on the effect of DP on biomass recalcitrance have proved that it is contributing factor and its variation can influence the LCB digestion process (Nazhad et al. 1995).

2.4.1.3

Effect of Pore Volume, Particle Size, and Surface Area

The enzyme absorption on the biomass substrate is necessary for its degradation by enzymatic systems. This makes the surface area of biomass which is accessible to the digestion system, an important factor in LCB recalcitrance. The overall accessible surface area (ASA) depends on the physical properties of LCB such as porosity, particle size, and pore volume. For example, an increase in the pore volume or decrease in the particle size can lead to an increase in ASA (Zhao et al. 2012a). Several studies have proven the effect of ASA on LCB recalcitrance; however, there

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is a limit to reduction in particle size after which the digestion does not improve. This particle size limit was found to be 350–590 μm in LCB (Chang et al. 1997; Moniruzzaman et al. 1997; Wen et al. 2004). The other factors that can influence the ASA and eventually LCB recalcitrance are pore volume and porosity. The total surface area (TSA) of LCB is a function of two components: (i) the external surface area of LCB which depends on the length and width of the section and (ii) internal surface area which is majorly affected by the number of substrate pores, cracks, and the size of the lumen. For cellulose, the internal surface area is higher by one or two orders than the external surface area (Chandra et al. 2007; Chang et al. 1981). It has been proven by several studies (Huang et al. 2010) that the internal surface area is the most important factor towards LCB recalcitrance. For cellulose as a substrate, the enzymatic systems access it mainly through the pores, and about 90% of enzymatic digestion is controlled by the ASA of the pores (Wang et al. 2012). The increase in TSA usually increases the ASA of biomass. However, the increase in the pore areas will increase the TSA but the pore has to be of a size that can entrap the enzyme systems performing hydrolysis (Tanaka et al. 1988; Zhang and Lynd 2004). The untreated biomass samples usually have the micropores and macroporous type of pore structures and no pores less than 100 nm diameter is present in them. However, after pretreatment nanopores between 50 and 100 nm are observed in the biomass (Meng et al. 2015). From multiple studies, the rate-limiting pore size limit for LCB was estimated to be 5.1 nm below which the enzyme systems cannot access the pore’s internal surface area (Zhao et al. 2012a). The reason for this is the size of cellulase compositions obtained from an organism such as Trichoderma reesei lies in this size range (~5.1 nm). The methods to measure TSA and ASA are based on the quantity of cellulase absorbed on a substrate. BET (Brunauer-Emmett-Teller) is a classical method to measure the TSA by utilizing nitrogen absorption. The shortcomings of BET are changes on biomass surface characteristics due to drying and use of nitrogen for estimation that ring in difference than enzymes used for hydrolysis (Mansfield et al. 1999; Zhang and Lynd 2004). The other methods that can be used to calculate interior surface area of the biomass are mercury porosimetry, X-ray scattering, DSC (differential scanning calorimetry), and NMR (nuclear magnetic resonance) (Beecher et al. 2009).

2.4.2

Contribution of Chemical Components of LCB on Recalcitrance

2.4.2.1

Lignin

Lignin has been widely accepted as the most important factor that is responsible for the recalcitrance of LCB. Conventionally, lignin present in the LCB is removed by applying a variety of methods which has been observed to enhance biomass hydrolysis. From the structural aspect, lignin is present as a physical barrier that restricts the access of enzymes to the cellulose and hemicellulose present in the biomass

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(Chang and Holtzapple 2000; Zhao et al. 2012a). The LCC formed by lignin and carbohydrate complexes also limits the accessibility to the LCB components (Chabannes et al. 2001). It is observed that woody biomass due to its high lignin content requires a comparatively harsh treatment process for its valorization than the grass-based biomass. The crucial factors that determine the effect of lignin on LCB recalcitrance are the variety of monolignols present and their abundance, and type and degree of cross-linking among polysaccharides and lignin in the cell wall (Laureano-Perez et al. 2005; Zhu et al. 2010). The structural aspects of lignin have a crucial function in determining the extent of biomass hydrolysis (Chandra et al. 2007). Some studies (Rollin et al. 2011) also suggest that removal of lignin does not necessarily imply that after this LCB would be more accessible to the enzymatic digestion system. Delignification certainly increases the rate of biomass hydrolysis; however, its removal does not ensure enhanced access to the cellulose substrate. An intriguing observation about the lignin in LCB is that it can absorb cellulase enzymes irreversibly. However, this ability to absorb cellulase depends on the biomass source and composition of lignin (Pan 2008; Sutcliffe 1986). Enzyme cellulase binds the lignin with electrostatic and hydrophobic interactions and the binding is spontaneous which was observed from thermodynamic studies. The time required for adsorption of cellulase molecules to lignin was estimated to be 3 h during studies which is much higher than the time required for cellulase adsorption to the pure cellulose (~60 min). The free energy change (ΔG
) of cellulase adsorption on lignin is 29.9 to 37.1 kJ/mol which shows that the binding is a result of physical adsorption. The effect of temperature is not significant in the case of cellulase adsorption to lignin as it is to cellulose; however, the ionic strength of the salts can decrease the cellulase binding to lignin. The adsorption of cellulase enzyme onto the lignin is in the range of 4.87–15 mg/g of lignin which was calculated by Langmuir and Freundlich isotherms (Nakagame et al. 2011). Due to the complex nature of LCB, it is difficult to determine whether these interactions have a significant role in the non-productive binding of the enzyme or not. Apart from lignin, hemicellulose and proteins are also present in the cell wall which also contributes to the LCB recalcitrance.

2.4.2.2

Hemicellulose

Hemicellulose as a component of LCB also contributes to the recalcitrance by acting as a physical barrier. Removal of hemicellulose seems to improve the digestibility of LCB (Zhao et al. 2012a). Xylan, which is present abundantly in hardwoods, can affect the recovery of sugars after hydrolysis and the amount of enzyme required (Bura et al. 2009). The addition of enzyme hemicellulases during enzymatic digestion has also been observed to enhance the hydrolysis of cellulose. This is due to the action of hemicellulases (xylanase) on the hemicellulose which degrades to form xylose, arabinose, and the low molecular weight lignin fragments that are associated with them (Mansfield et al. 1997; Yoshida et al. 2008). However, compared to lignin, the effect of hemicellulose on biomass recalcitrance is not that significant as it is less complex and easier to remove by both the pretreatment and hydrolysis system

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(Zhu 2011). Cellulose and hemicellulose are present as holocellulose in the LCB. In recent years, the studies related to the properties and valorization of holocelluloses have increased. For example, Park et al. (2017) in their work studied the effect of hemicellulose content on holocellulose fibril properties. It was observed that with the decrease in the hemicellulose content, crystallinity index, and specific surface area of holocellulose nanofibrils increased, whereas the filtration time and diameter decreased. However, increase in hemicellulose content showed a rise in elastic modulus and tensile strength of nanopaper sheets formed from holocellulose nanofibrils. In another study by Huang et al. (2020), water-soluble ammonium polyphosphate and methyl trimethoxysilane modified bamboo residue were used to obtain the holocellulose nanofibrils. These nanofibrils were further used to prepare the aerogels using the freeze-drying process. The formed aerogels were flexible, soft, and has a 3D network-like structure. The formed aerogels also have good flame retardant and hydrophobic properties. The thermal conductivity of this aerogel was 0.039 W/m K which shows its applications for preparing thermally insulative materials. The removal of hemicellulose by the enzymatic or chemical process can reduce the biomass recalcitrance and enhance the digestibility of cellulose. The backbone of hemicellulose in LCB is present in a highly acetylated form. These acetyl groups amount to 1–6% of LCB which varies in different plant species. The acetyl moieties are considered to contribute significantly to biomass recalcitrance (Grohmann et al. 1989; Peng et al. 2011). The attachment of acetyl moieties to the cellulose decreases its enzymatic hydrolysis. From several studies, it was observed that deacetylation of LCB can increase its hydrolysis rate. This is because acetyl moieties can interfere with the recognition sites of enzymes which can reduce the rate of hydrolysis. It may also affect the hydrogen bonds-based binding of cellulose with cellulase. Acetyl moieties on cellulose can also increase the chain diameter, which provides the steric hindrance to the enzymatic systems (Pan et al. 2006; Zhu 2005).

2.4.2.3

Proteins in the Cell Wall

The proteins present in LCB also have an effect on its enzymatic hydrolysis. The functional class of proteins present in the plant cell wall are: (i) proteins acting on carbohydrates, (ii) oxidoreductase proteins, (iii) proteases, (iv) proteins with interaction domains, (v) proteins possibly involved in signaling, (vi) structural proteins, (vii) proteins related to lipid metabolism, (viii) miscellaneous proteins. Some examples of these proteins are glucoside hydrolases, peroxidases, and expansins. The type and abundance of cell wall proteins vary with biomass source and cell types in the plant (Albenne et al. 2013; Cassab 1998). They either contribute to the LCB hydrolysis or discourage it. The endogenous enzymes in the plant cell wall such as hemicellulase (endoxylanases), cellulase (endoglucanase), and enzymes that play a role in debranching of the polysaccharides (ferulic acid esterase) have a positive effect on biomass hydrolysis (Lagaert et al. 2009). The proteins that have a negative effect on biomass hydrolysis inhibit the activity of the biomass-degrading enzymes

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present in the cell wall. Some examples of these enzymes are xylanase inhibitors and polygalacturonase inhibiting proteins. However, the effects of these proteins can be overlooked because these proteins are active in freshly harvested biomass and during processing and storage of biomass, these proteins undergo denaturation and degradation (Han and Chen 2007; Zhao et al. 2012a).

2.4.3

Combined Effect of Physical Properties and Chemical Compositions on Recalcitrance

In the previous sections, the role of physical properties and chemical composition on the LCB recalcitrance was discussed. The chemical structures have their own set of interactions which enhances the recalcitrance. However, combined interactions of these physical properties and chemical compositions play a huge role and are interrelated. The removal of a chemical component would certainly affect the physical properties of the cell wall, whereas varying the physical properties would reflect upon the chemical compositions of LCB. For example, the removal of lignin and hemicellulose increases the porosity of biomass which has a direct effect on the ASA for enzymes (Mansfield et al. 1999). Similarly, a decrease in DP and crystallinity of cellulose results into particle size reduction and increased specific surface area. The factors that affect the accessibility to biomass can be categorized into two factors: (i) Direct and (ii) Indirect factors. In direct factors, enzymatic digestion system interacts with the ASA of LCB, which makes it one of the most important factors for enzyme hydrolysis (Jeoh et al. 2007). Indirect factors are related to the structure of the biomass. Examples of these factors are particle size, pore size and volume, DP and crystallinity of cellulose, and chemical composition of LCB. However, in some cases, the indirect factors can also affect the enzyme hydrolysis process directly. An example for this would be the cellulase adsorption by the lignin which reduces the effective enzyme concentration (Zhao et al. 2012a). It is clear from the discussion that recalcitrance in LCB is not the product of a single structural or compositional factor. The interactions between chemical components result in the formation of rigid and compact LCB structures which makes its utilization difficult in the natural form. A solution to counter this rigidity is a process called pretreatment. Pretreatment is the most significant process in the lignocellulose biorefineries, and there has been a lot of research on this process to utilize the biomass effectively. Pretreatment is a key that opens the full potential locked in the LCB for its successful valorization in biorefineries which will be discussed in detail in the next section.

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Pretreatment: Introduction and Its Role

The LCB as biomass is resistant to breakdown by biological and chemical agents due to its recalcitrance, as discussed in the previous section. Pretreatment processes are required to remove this recalcitrance so that LCB can be utilized optimally in biorefineries. It influences the recalcitrance by processes such as hemicellulose degradation, lignin sheath removal, reduction in DP, and crystallinity of cellulose present in the fibers. The pretreatment process enhances the accessibility to chemicals and enzymes towards the digestion of LCB. Different biomass sources display the variation in the physical properties and composition which requires the selection of different pretreatment processes. Different pretreatment strategies induce different effect on LCB and are utilized according to their special features (Baruah et al. 2018). The goals associated with the selected pretreatment processes are (i) minimum degradation of sugars capable of fermentation, mainly the ones obtained from hemicelluloses, (ii) production of biomass that is able to give a high sugar yield on enzymatic hydrolysis, (iii) minimization of inhibitor formation such as phenolic compounds from lignin degradation, acetic acid from hemicellulose, hydroxymethylfurfural (HMF), furfurals, and other acids such as levulinic acids, formic acid from sugars as shown in Fig. 2.7, (iv) removal and recovery and of lignin for successful valorization, (v) low cost and less energy requirements, (vi) hemicelluloses removal from cellulose, (vii) removal of acetyl groups that interfere with lignocellulose recognition by enzymatic systems, (viii) reduction in particle size and increase in porosity of the biomass, (ix) enhancement of pore size of the components for efficient hydrolysis, and (x) minimization of downstream processes involved in the product recovery, which otherwise will make the bioproduction costly (Chiaramonti et al. 2012; Ramos et al. 2019; Zhao et al. 2012b). These goals are interrelated in nature meaning that changing one of these can affect the others. For example, removal of hemicellulose in LCB can also improve the pore size of the biomass which will enhance the hydrolysis yield. The increase in the porosity also affects the pore size which is a limiting factor for biomass hydrolysis. The selection of a particular pretreatment process for biomass depends on the type/source of biomass origin and the product required (Chiaramonti et al. 2012). The assessment of a particular pretreatment for a biomass can be done by parameters such as (i) the accessibility of water-washed or unwashed biomass to hydrolyzing systems, (ii) the total recoverable amount of carbohydrates released (monomeric and oligomeric units present in solid or liquid phase), (iii) the fermentation ability of hydrolysates obtained after pretreatment which can be characterized by product yields and microbial growth during fermentation, (iv) the formation of useful by-products for the synthesis of other value-added products besides the main intended product (Ramos et al. 2019). Pretreatment is one of the most important processes in biorefineries and needs strategic planning as it determines the economic viability of the process. The energy balance, chemical consumption, production of enzymatic and fermentation inhibitors, and environmental effects are some other factors that are considered before selecting the appropriate pretreatment process

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Fig. 2.7 Schematic representation of by-products and inhibitors produced from lignocellulosic biomass valorization and the respective products that can be obtained with them

(Kapanji et al. 2019; Ng et al. 2019). The selection of the pretreatment also influences the upstream and downstream systems that are applied during the processing of LCB. For example, in upstream processes, the choice of the pretreatment process can influence the harvesting and storing conditions of biomass and the chemical and techniques utilized for it. Similarly, the pretreatment process applied can also influence the enzyme loading needed for hydrolyzing the substrate, downstream processing utilized for recovery and isolation of the product, and wastewater generated from the process that can be recycled (Bensah and Mensah 2013; Harmsen et al. 2010). The types of pretreatments utilized in biorefineries are physical, chemical, physicochemical, biological, and combined pretreatments. Physical pretreatment processes, as the name suggests, use the physical and mechanical processes to bring down the recalcitrance of LCB. It reduces the particles size and increases ASA for hydrolysis. These methods are generally used in combination with other pretreatment methods. Some examples of physical pretreatments are milling, radiation pretreatment, extrusion, freezing, and pyrolysis (Zhao et al. 2012b). Chemical pretreatment uses the chemicals for pretreatment of LCB under defined conditions. The mechanism of chemical pretreatment varies with the type of chemical utilized. Some examples of chemical pretreatment are acid pretreatment,

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alkaline pretreatment, sulfite pretreatment, oxidative pretreatment, ozonolysis, organosolv pretreatment, and ionic liquid-based pretreatment (Bhardwaj et al. 2020a; Kumar et al. 2018; Soltanian et al. 2020). Physiochemical pretreatment methods use physical and chemical methods in combination to increase the efficiency of the pretreatment. Some examples are ammonia fiber explosion (AFEX), steam explosion, liquid hot water pretreatment, wet oxidation, and ultrasonication (Chiaramonti et al. 2012). Biological pretreatment processes use the application of biological agents such as microorganisms or enzymes to reduce the recalcitrance possessed by LCB. Some examples of this pretreatment are fungal, bacterial, enzymatic, and microbial consortium-based pretreatment (Kumar et al. 2018; Bhardwaj and Verma 2020; Bhardwaj et al. 2020b; Soltanian et al. 2020). The last category in pretreatment processes would be the combined pretreatment. The different pretreatment strategies have their own shortcomings and to reduce this, two or more pretreatments could be applied to the biomass. This will help in increasing the efficiency of the pretreatment by improving sugar production, less time consumption, and less production of inhibitors. It also helps in improving the economic viability and product yield. Some examples are combined CO2 and steam explosion pretreatment, combined biological and dilute acid pretreatments, combined alkali and ionic-liquid pretreatment, and microwave-assisted alkali pretreatment (Kumari and Singh 2018). This information about the main products that are generated in different pretreatment processes from the individual biomass component can supplement in choosing the right pretreatment method for a particular product. For example, in cellulose, acid hydrolysis generates glucose; pulping, bleaching, and enzymatic hydrolysis yield cellulose nanocrystals whereas pulping, bleaching, and mechanical processing produces cellulose nanofibers. For hemicellulose, acid hydrolysis generates monosaccharides that constitute xylose arabinose, etc., autohydrolysis and mild acid hydrolysis generate oligosaccharides whereas partial acid hydrolysis and functionalization produce hydrocolloids. For lignin, oxidation forms vanillin, acid or alkaline hydrolysis produces phenolic acids, and sulfite pulping produces lignosulfonates (Ramos et al. 2019). Based on the discussion above, it can be concluded that pretreatment is a crucial step in LCB valorization in biorefineries. Some of the examples of pretreatment used for lignocellulosic biomass are presented in Table 2.1. along with the methodology and sugar yield obtained. In recent years, the development of a low-cost and efficient pretreatment process has been researched thoroughly and the pretreatment strategies available at present allow excellent performance in biomass utilization. However, these strategies still have some obstacles that are needed to be solved such as high energy and chemical demands, waste production, efficient recovery of LCB components, and production of inhibitors. The role of pretreatment in biorefineries is huge and contributes largely to the cost associated and therefore requires careful selection. Selecting the right pretreatment process is not an easy task and requires the understanding of economic and sustainability aspects, biomass used, products required, and optimum conditions for operation.

Supercritical CO2 explosion Ammonia fiber explosion Alkaline hydrolysis Liquid hot water Ionic liquids

Corn stover

Organosolv

Steam explosion Ceriporiopsis subvermispora

Sitka spruce

Mandarin peel Japanese cedar wood

Sorghum bagasse Sugarcane bagasse Rice straw

Switchgrass

Pretreatment Acid hydrolysis

Substrate Rice hulls

Physicochemical pretreatment Biological pretreatment

Physicochemical pretreatment Physicochemical pretreatment Chemical pretreatment Physicochemical pretreatment Chemical pretreatment Chemical pretreatment

Type of pretreatment Chemical pretreatment 5g raw material, 165
C, 34.5 MPa, 60 min Ratio: 1.5:1 ammonia: biomass loading (w/w), 140
C, 20 min Ammonium hydroxide (28% v/v), 130
C, 1 h High pressure CO2, solid: liquid ratio: 1:12, 115
C, 60 min 1-ethyl-3-methylimidazolium acetate, 5% solid, 120
C, 5 h Ethanol: water 60:40 (H2SO4, 1%), solid: liquid ratio 1:10, 180
C, 60 min Hot popping: 150
C, pressure: 15 kgf/cm 8 weeks at 28
C, 70% moisture content

Methodology H2SO4 (0.3 %), solid: liquid ratio 5: 95, 152
C, 33 min

Choi et al. (2013) Wagner et al. (2018)

Glucose 45.1 g/100 g raw material; fructose 18.4 g/100 g raw material 35% and 5% conversion of holocellulose to methane with and without pretreatment, respectively

Glucose 38.6 g/100 g biomass, Ethylglycosides 12.5 g/100 g raw material

Glucose 450 g/kg rice straw

Glucose 30.43 g/L, xylose 9.82 g/L

Glucose 69.3 g/100 g glucan, xylose 67.7 g/ 100 g xylan Glucose 42 g/100 g raw material

Total sugar 37 g/100 g raw material

References de Moraes Rocha et al. (2011) Huisheng et al. (2013) Garlock et al. (2012) Chen et al. (2012) Gurgel et al. (2014) Poornejad et al. (2014) Bouxin et al. (2014)

Yield Glucose 47.2 g/100 g glucan; xylose 86.3 g/100 xylan

Table 2.1 Examples of different pretreatments processes applied to lignocellulosic biomass along with their methodology and respective yield

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Pretreatment-Induced Changes on LCB

LCB in its natural form shows recalcitrance towards enzymatic and chemical hydrolysis which is the basis behind the use of pretreatment in biorefineries. The pretreatment processes applied to LCB change its chemical composition and physical properties. Generally, pretreatment disrupts the physical barriers of the cell wall, removes lignin and hemicelluloses, alters the cellulose crystallinity, helps in swelling of pores, and increases porosity as shown in Fig. 2.1(b). The pretreated biomass due to these changes becomes different from the natural LCB which could be observed from variation in physical properties of kraft pulp before and after pretreatment as shown in Table 2.2. (Popescu et al. 2009). Physical pretreatment changes the crystallinity, particle size, and surface area whereas chemical pretreatment generally focuses on swelling of cellulose and lignin removal (Mussatto 2016). Considering physical pretreatments, the ball milling process performs the size reduction (~78%), decreases cellulose crystallinity (from 74.2% to 4.9%), and DP, with ASA of~ 2.13–2.26 m2/g (Yeh et al. 2010; Zhu 2011). Another physical pretreatment based on radiation-based, significant hemicellulose (xylan increase 13.9–16.3%) and lignin removal (26.3–6.8%) is observed. In terms of physical properties, it enhances the ASA and decreases the cellulose crystallinity (27.3–24.6%) and DP (1920–170). The ball milling process does not degrade the components of biomass; however, physical pretreatment affects the chemical composition of the LCB (Shin and Sung 2008; Zheng et al. 2009). Ultrasound-based physical pretreatment modifies the structure by cavitation and breaks LCC complexes. For the chemical pretreatments, dilute acid shows significant removal of Kalson lignin and partial degradation of hemicellulose (xylan 8.1–5.5%) which almost disrupts the structure completely. It removes almost all the acetyl groups too (up to 90%), decreases crystallinity (up to 4.4%), and DP (up to 84.7%) of cellulose. In concentrated acid hydrolysis, swelling of cellulose, partial hydrolysis of hemicellulose, and coalescence and condensation of lignin are observed. In alkali pretreatment, significant removal of lignin (>50%) with partial degradation of hemicellulose disrupts the physical structure of LCB. It also removes the acetyl groups completely, increases ASA significantly, with notable and some decrease in the cellulose crystallinity and DP, respectively. In supercritical CO2 pretreatment, partial hydrolysis of hemicellulose, and an increase in ASA and pore volume are Table 2.2 The change in physical properties of unbleached and bleached kraft pulp (Popescu et al. 2009). Properties Viscosity (L/g) Density (bulk) (m3/kg) Water retention value (kg/kg) Elastic modulus (N/mm2) Tensile strength (kN/m) Kappa number

Unbleached kraft pulp properties 1.10 0.0019 1.59 2420 1.92 26.8

Bleached kraft pulp fibers 0.728 0.0018 1.43 2129 1.74 3.2

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observed. In oxidative delignification, significant removal of the lignin (~50%) is seen with almost complete removal of hemicellulose. It also removes acetyl groups partially, increases ASA significantly, decreases DP of cellulose. Ionic liquid pretreatment partially removes the lignin (15.6–36.3%) (Sathitsuksanoh et al. 2014; Zhao et al. 2012b). It significantly increases ASA, completely removes acetyl groups, converts crystalline cellulose to amorphous, and shows a decrease in cellulose DP (1198–812). Organosolv pretreatment (organic acid-based) shows significant lignin removal (60–90%) and almost completely degrades hemicellulose. It shows a significant increase in ASA with some decrease in cellulose DP. Physiochemical pretreatments such as stream explosion almost completely remove hemicellulose (80–100%) and partially removes lignin (26–50%), however, cause significant changes in redistribution and structure. For physical properties, it significantly removes the acetyl groups and increases ASA significantly. In liquid hot water pretreatment, partial lignin removal (35–60%) happens with significant changes in structure and redistribution of the components with significant and almost complete degradation of hemicellulose (90%). For biological pretreatments, in fungal degradation, partial removal of lignin (13–41%) and good recovery of hemicellulose (81%) is observed with some cellulose degradation. It enhances the ASA and reduces the cellulose DP to some extent. For enzymatic pretreatment, lignin and hemicellulose degrading enzymes such as laccases, peroxidases, and xylanase comes into play that potentially degrades the lignin and hemicellulose in the LCB and increase ASA for its access to the chemical and enzymatic hydrolysis system (Zhao et al. 2012b). Figure 2.8 shows the different physical, chemical, physiochemical, and biological pretreatment methods available for LCB and the changes in biomass induced by them. Wheat straw as a lignocellulosic substrate when undergoing thermal pretreatment, several changes are observed in the LCB composition. At 180
C temperature, an increase in the cellulose content is observed, from 34.5% to 47%. At this temperature, a reduction in the hemicellulose content was observed. The reason could be the generation of hydronium ions which could have degraded the materials, generating simple compounds from complex LCB components. Significant removal of lignin was also observed at this temperature. The results also show an increase in crystallinity from 39.7% to 48.5%. The reason for the increase in crystallinity could be a decrease in hemicellulose, lignin, and amorphous cellulose content in the biomass. The fracture of lignin and hemicellulose structure also changes the chemical structure of the biomass, resulting in the change in chemical bonding which was found by Fourier-transform infrared spectroscopy spectra (FTIR) (Rajput and Visvanathan 2018). Bagasse which is obtained from the hybrid of wild and commercial sugarcanes (energy cane bagasse) when undergoes the pretreatment with ionic liquids such as [EMIM][OAc] shows a significant removal of lignin (~32%) with slight xylan and glucan degradation (14% and 8.8%, respectively). The treatment also improved the enzymatic digestibility of the energy cane bagasse. After pretreatment, a disordered and loose structure of biomass was observed from scanning electron microscope (SEM) observations. The crystallinity of cellulose in the biomass also decreased from 56.28% to 24.52% (Qiu et al. 2012). Rice straw

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Fig. 2.8 Schematic representation of changes induced after pretreatment of lignocellulosic biomass

when pretreated with alkali shows a significant change in the biomass composition. At 12% NaOH pretreatment for 1 h, the glucan content in the biomass increased from 34.99% to 64.96% whereas the xylan content decreased from 21.55% to 9.07%. Lignin content and extractives also decreased from 24.28% to 4.96% and 16.05% to 14.85%, respectively, during these conditions. The effect of pretreatment in changing the physical properties and chemical composition of the biomass is quite evident and is proven with several studies on different substrates (Kumar and Verma 2019; Harun and Geok 2016). Each pretreatment applies a unique set of changes to the biomass. The innovative pretreatment methods that allow better accessibility to biomass and minimize the waste is a topic of interest from a research perspective and is expected to grow more in the future to make a zero-waste biorefinery. The biorefinery-based production is although classified into various types depending on the feedstocks utilized, pretreatment processes involved, and types of intermediates generated. However, the biorefineries can be divided into different phases such as phases I, II, and III. Phase I biorefineries use single feedstock and produce a single product (e.g., pulp and paper, biodiesel) whereas phase II biorefineries utilize a single substrate to generate multiple products (e.g., bioethanol production). Phase III biorefineries use various feedstocks to produce multiple type of products (LCB biorefinery, whole crop biorefinery). The innovation that has been introduced in the LCB biorefinery in recent years is also starting to gain industrial attention. This has led to a variety of LCB biorefinery products adapted by industries (e.g., succinic

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acid, ethanol, and lactic acid) which projects new prospects for this area in the coming future with unlimited possibilities (Takkellapati et al. 2018).

2.7

Conclusion

The rapid consumption of natural petroleum-based reserves to obtain fuels and materials is leading to their quick depletion and huge environmental pollution. It is high time to search for alternative natural substrates to obtain these products which will reduce the dependence on petroleum reserves and pollution caused by them. In recent decades, lignocellulosic biomass from wood has been identified as a suitable substrate for this purpose which can replace the conventionally used petroleumbased raw materials. However, the inherent resistance or recalcitrance of the biomass protects it from biological and chemical agents and suppresses its utilization at full potential. To counter this, pretreatment processes are used in biorefineries that make the LCB more susceptible to the degradation process and facilitate its optimal utilization in biorefineries. The pretreatment processes are of different types that change the physical properties and chemical composition of biomass depending on the biomass used and the pretreatment process and conditions. The ongoing research is towards the development of innovative pretreatment methods with high efficiency for pretreatment that can increase the economic value of LCB-based biorefineries and industries. Acknowledgment R.R is thankful for the junior research fellowship from Department of Biotechnology, Govt. of India (DBTHRDPMU/JRF/BET-20/I/2020/AL/07). P.D. is grateful for the Ramalingaswami fellowship and the research grant obtained from the Department of Biotechnology, Govt. of India (BT/HRD/35/02/2006). The authors are grateful to School of Biochemical engineering, Indian Institute of Technology (IIT-BHU), Varanasi for their kind support which made this work possible. Competing Interests All the authors declare that they have no competing interests.

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Chapter 3

Understanding Biomass Recalcitrance: Conventional Physical, Chemical, and Biological Pretreatment Methods for Overcoming Biomass Recalcitrance Saurabh Kumar, Richa Prasad Mahato, Kuldeep Gupta, Pritam Bardhan, Muzamil Ahmad Rather, Manabendra Mandal, and Rupam Kataki

Abstract Pretreatment of lignocellulosic waste is one of the costliest phases in transforming cellulosic material into fermentable sugars. It represents one-third of the overall process cost, and about 90% of the dry weight constitutes cellulose, hemicellulose, lignin, and pectin. Hydrogen bonds and some covalent bonds bind the carbohydrate polymers to lignin firmly. The existence of lignin in lignocelluloses barricades the plant cell against the breakdown action by fungi and bacteria. The purpose of the pretreatment procedure is to disrupt the crystalline phase of cellulose and disintegrate the lignin structure, improving the porosity of the lignocellulosic material. It further provides acids and enzymes access to hydrolyze cellulose by expanding the porosity of the lignocellulosic material so that it readily attacks to break down the cellulose. Pretreatment is therefore done: (i) to facilitate hydrolysis for the formation of sugars, (ii) to keep away the decaying or waste of carbohydrates, (iii) to prevent the creation of by-products that hinder the hydrolytic activities and fermentation that follow, and (iv) make it economical. Numerous pretreatment protocols are employed for treating biomass to overcome the problem faced during pretreatment. In this S. Kumar Bioprospection and Product Development Division, CSIR-Central Institute of Medicinal and Aromatic Plants, Lucknow, Uttar Pradesh, India R. P. Mahato Department of Microbiology, Kanya Gurukul Campus, Gurukul Kangri University, Haridwar, Uttarakhand, India K. Gupta · P. Bardhan · M. A. Rather · M. Mandal Department of Molecular Biology and Biotechnology, Tezpur University, Napaam, Tezpur, Assam, India R. Kataki (*) Department of Energy, Tezpur University (A Central University), Napaam, Tezpur, Assam, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_3

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study, we have dealt with various pretreatment adopted against lignocellulosic biomass, which is a measure of their potential as feedstock for biofuels. Keywords Lignocellulosic waste · Pretreatment · Cellulose · Lignin

Abbreviations AFEX HMF HPSE OLP

3.1

Ammonia fiber explosion 5-hydroxymethylfurfural High-pressure steam explosion Oxidative lime pretreatment

Introduction

Rapid population expansion, drop-in biodiversity, and natural resource switches in pollution and climate (Correa et al. 2019) have prompted academics and policymakers to focus on sustainable energy policies to reduce greenhouse gas emissions (Andrée et al. 2017). Furthermore, the depletion of fossil deposits is occurring faster due to an increased demand for fossil fuels due to global industrialization and motorization (Perin and Jones 2019). Fossil fuels are still the world’s principal energy source, responsible for more than 88% of primary energy consumption (Li et al. 2019). Their burning releases greenhouse gases, mainly CO2 (Alalwan et al. 2019; Goswami et al. 2020). As a result, substituting fossil fuels with renewable energy sources allow us to fix these problems by reducing global temperature rise (Scaramuzzino et al. 2019; Goswami et al. 2021). Renewable energy is the proportion of renewable energy to total primary energy consumption, which comprises the equivalent of direct energy from hydroelectric sources (excluding storage through pumping), geothermal, solar, wind, tidal, and wave energy other factors. Power is provided by solid biofuels, biodiesel, different liquid biofuels, biogas, and the renewable component of municipal garbage. On the other hand, biofuels are fuels generated directly or indirectly from biomass (OECD 2019; Goswami et al. 2022a, b). Biomass is the largest and most important renewable energy source today, after coal, oil, and natural gas (Azevedo et al. 2019; Kumar and Verma 2020a). It can generate various energy sources because the carbon dioxide released during combustion is equivalent to the amount absorbed by the plant throughout photosynthesis, so biomass can be termed carbon neutral (IbarraGonzalez and Rong 2019). Lignocellulosic biomass, the world’s most cost-effective and highly renewable natural resource (Hernández-Beltrán et al. 2019; Agrawal and Verma 2020; Goswami et al. 2022c) is the most significant potential feedstock for bioenergy production.

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Pretreatment and Hydrolysis of Biomasses: Overview

Pretreatment or fractionation is a valuable technique for changing the structure of lignocellulosic biomass (Fig. 3.1) and making the holocellulose (cellulose + hemicellulose) bioavailable for bioconversion (Rajendran et al. 2018; Bhardwaj et al. 2021). Pretreatment’s primary goal is to break down the cell wall’s physical barriers, allowing it to depolymerize and lower cellulose crystallinity (Karimi and Taherzadeh 2016; Kumar et al. 2020). Depending on the type of biomass, pretreatment has been recognized as a crucial stage in the bioconversion of lignocellulosic biomass for utilization in biorefineries (Mupondwa et al. 2017). Pretreatment must be cost-effective, as operational and capital expenditures could account for more than 40% of total processing costs (Bhutto et al. 2017). There have been various studies on biomass pretreatment methods, which can be divided into four categories: (1) mechanical processes; (2) chemical processes; (3) physicochemical processes; and (4) biological processes, which employ microbial consortia or enzymatic means (Fig. 3.2).

3.2.1

Physical/Mechanical Pretreatment

Physical approaches can improve lignocellulosic biomass’s pore size and surface area while reducing cellulose crystallinity and polymerization. Milling, sonication, mechanical extrusion, and irradiation are physical pretreatment. Mechanical pretreatment minimizes the size of the particles by milling, chipping, or grinding (Kumari and Singh 2018). Pretreatment with mechanical equipment is a well-known way of enhancing biogas production, but due to its high energy requirements, it is still a costly method (Veluchamy and Kalamdhad 2017).

Fig. 3.1 (a) The primary cell wall, (b) secondary cell wall of the plant’s cell wall

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Fig. 3.2 Bioavailability of lignocellulosic biomass after pretreatment

To improve lignocellulosic biomass’s accessible surface area and porosity, it must first be reduced in size, lowering cellulose crystallinity and improving the accuracy of the following processing phase and the overall production line (Rodriguez et al. 2017). Mechanical pretreatment benefits from not producing any secondary inhibitory compounds, implying that they might be used for methane synthesis or other bioprocesses (Dahunsi 2019a). Mechanical processing of six distinct lignocelluloses produced structural material degradation and enhanced methane output by 22%. However, excessive lignocellulosic biomass size reduction can impair methane generation efficiency (Kumari and Singh 2018). Operational variables such as particle size, mechanical pretreatment processing time, and mechanics all affect the performance of the operation in terms of cost energy. According to the literature, there is no standard particle size acceptable for the methanation process, and it varies depending on the type of substrate and the biomethanation procedure. Dumas et al. (2015) found no correlation in wheat straw biogas production with particle sizes ranging from 0.7 to 0.2 mm, and that methane output did not go up at 0.048 mm. The impacts of various particle sizes of wheat straw, rice straw, Mirabilis leaves, cauliflower leaves, Ipomoea fistulosa leaves, dhub grass, and other agricultural residues were investigated at 0.088, 0.40, 1.0, and 6.0 (Sharma et al. 1988). They also stated that the biogas yield difference between 0.4 and 0.088 mm was not

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statistically significant. De la Rubia et al. (2011) found that particle size 1.4–2.0 mm produced the maximum methane yield in sunflower oil cake, 17% greater than particle size 0.355–0.55 mm. Compared to both straws’ particle size of 0.75 mm, the particle size of 0.30 mm of wheat and rice straw did not exhibit any substantial improvement in methane yield, according to Chandra et al. (2015). Both particle sizes, however, increased as compared to the untreated particle size of 1.5 mm. Tsapekos et al. (2015) studied six different mechanical pretreatments that produced particle sizes ranging from 0 to >20 cm. They found that mechanical pretreatment that made particle sizes greater than 20 cm, which accounted for 33% and 41% of the overall biomass, did not yield over untreated biomass. The proportion of biomass with more than >20 cm particle sizes was only around 22% of the total biomass; the methane output was significantly higher and enhanced by 25%. Herrmann et al. (2012) revealed a substantial connection between particle length and methane yield when evaluating different particle lengths of similar agricultural material. Two particle sizes, 2 mm and > 20–100 mm, on various substrates, including softwood, hardwood, and cotton, were studied by Krause et al. (2018) and discovered that lowering particle size enhanced methane output. On the other hand, particle reduction processing time is essential for energy and the efficiency of lignocellulosic biomass bioconversion (Bhutto et al. 2017). Rodriguez et al. (2017) evaluated 30 and 60 min of mechanical pretreatment and found that 60 min resulted in a 21% increase in methane output, while 30 min did not. The milling equipment’s speed must be addressed in order to reduce the cost of energy utilized in mechanical pretreatment (Tsapekos et al. 2017) investigated the effect of different milling speeds, ranging from 200 to 1200 rpm, on CH4 production through meadow grass and found that the milling rates studied made no difference. Treatments resulted in a 27% enhancement in CH4 output compared to untreated material. There was also a drop in methane yield when particle sizes were greater or lower than the range of 2–5 mm (Chandra et al. 2015; Menardo et al. 2012; Dell'Omo and La Froscia 2018). According to Menardo et al. (2012), increasing the particle size of wheat straw to 2 mm increased CH4 production by 83%. Similarly, at particle sizes of 5 mm and 20 mm, barley straw increased methane output by 54% and 41%, respectively (Menardo et al. 2012). Because of the differences in particle size, milling time, and milling speed, the optimal mechanical pretreatment technique appears to depend on the type of lignocellulosic material, and no general strategy can be advised. Waste liquid streams from pulp and paper factories are attracting a lot of attention in Northern Europe. On steam-pretreated birch wood, Lamb et al. (2019) employed ultrasonication, a Fenton-like reaction, or a combination of the two to see if a favorable influence on biomethane generation could be obtained. A Fenton reaction uses iron ions and hydrogen peroxide to oxidize organic pollutants like inhibitors and poisonous compounds that can harm microorganisms. Birchwood was steam processed for 10 min at 210  C in the study. After that, the STEX material was ultrasonicated for 2 h at a pH of 4, with or without the addition of hydrogen peroxide and FeCl3. The findings imply that a strong ultrasonication combined with a

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Fenton-like treatment negatively influenced biogas generation, but a softer treatment increased production. They led to a shorter residence time to get near the maximum BMP, which has a significant impact on reactor size (or biogas capacity), which is the most expensive part of a biogas plant.

3.2.1.1

Milling

Milling can make lignocelluloses more accessible to cellulases based on the intrinsic cellulose ultrastructure and crystallinity degree. Cellulases are enzymes that catalyze the breakdown of cellulose; however, for the enzymes to operate optimally, substrate supply must be increased. Before introducing lignocellulosic materials to enzymatic hydrolysis, they should be milled and size reduced. Colloid mills, dissolvers, and fibrillation are suitable for wet materials, while hammer mills, extruders, cryogenic mills, and roller mills are suitable for dry materials. Ball milling can be used for both wet and dry materials. Hammer milling is the best pretreatment method for wastepaper. By lowering the crystallinity and size of the material, milling promotes enzyme degradation. Particle size reduction of near 0.2 mm can be achieved through milling and grinding. Particle size reduction is possible to a degree; then, particle size reduction does not influence the pretreatment procedure. Small particle sizes corn Stover of 53–75 m is more productive than corn stover with large particle sizes, 475–710 m. Because of the variation in particle size, productivity has a considerable impact on the pretreatment process. (Bondy et al. 1998; Taherzadeh and Karimi 2008; Walpot 1986) found that ball milling reduces the crystal index from 4.9% to 74.2%, making it pretty appropriate for straw saccharification under mild hydrolytic circumstances with more fermentable sugars yield. Grinding and milling procedures are not the only types of physical preparation. Falls et al. (2019) investigated the impact of “shock treatment.” For example, the method depended upon the effects of an immediate shock wave on material when treated with enzymatic hydrolysis. They investigated various lignocellulosic materials in the study, including bagasse, corn stover, poplar wood, sorghum, and switchgrass. One of the objectives was to examine the digestion of oxidative lime pretreatment (OLP) substrates by glucan enzyme after ball milling and shock treatment. All substrates responded better to shock treatment than ball milling at a hydrolysis time of 24 h. Ball milling outperformed shock treatment when the residence duration was increased. There was a significant improvement in digestibility compared to solely OLP substrates, leading to increased glucose production (Galbe and Wallberg 2019). Milling is a technique that can be employed in conjunction with enzymatic hydrolysis to improve hydrolysis results. It is possible to carry out mechanical action, mass transfer, and enzymatic hydrolysis in parallel when two techniques are coupled. Compared to biomass pretreatment without milling technology, small

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enzyme feeding is required in a bill mill reactor. Numerous ball beads contribute a critical part in cellulose hydrolysis, and a 100% hydrolysis rate can be obtained.

3.2.1.2

Irradiation

Microwave irradiation is a commonly used irradiation technology for plant biomass processing. This pretreatment approach has several advantages, including the convenience of use, higher heating capacity, quick processing time, minimum inhibitor generation, and lower energy use. In 1984, a group of researchers from Kyoto University in Japan reported the first case of microwave irradiation in a closed container. In the presence of water, they microwaved sugarcane bagasse, rice straw, and rice hulls. Glass jars of 50 mL, 2450 MHz energy, and 2.4 kW microwave irradiation were utilized for microwave therapy (Azuma et al. 1985). Traditional pretreatment procedures used high pressures and temperatures. In response to the extreme heat, chemical connections between lignocellulosic materials break down, boosting substrate availability for enzymes. In traditional healing processes, lignocellulosic material is heated to temperatures between 160 and 250  C using an injection of high-pressure steam or indirect heat. However, to avoid temperature gradients, lignocellulosic material must be crushed into small particles. Microwave is a suitable alternative for avoiding significant temperature gradients since it evenly distributes heat and prevents lignocellulosic material breakdown into humic acid and furfural. For effective degradation, microwave irradiation is paired with a mild alkali agent. Alkali and irradiation combination pretreatment of switchgrass resulted in 70–90% (Hu and Wen 2008). Microwave irradiation is done at a high temperature, requiring using sealed containers to attain that temperature. Microwave demonstrates three properties: penetration, reflection, and absorption. Penetration, reflection, and absorption are three characteristics of the microwave. Microwaves penetrate through glass and plastic, acquiring energy from water and biomass, whereas metals reflect them. Microwave reactors can be classified into two sorts based on these characteristics: those that allow microwaves to flow through and those that reflect them. The first type of microwave reactor is glass or plastic, whereas the second type is steel. Microwaves can enter the reactor through quartz windows, which are positioned in the reactor. For microwave irradiation pretreatment, a closed, sealable, pressureresistant glass tube container with a Teflon gasket can be utilized at a high temperature of 200  C. The temperature of the microwave is controlled and maintained using sensors. Due to their features like thermostability, corrosion resistance, and zero absorbance, Teflon-coated sensors are an ideal choice. Some scientists utilize Teflon vessels in microwave ovens because of their beneficial features (Komolwanich et al. 2014; Azuma et al. 1984). The size of a ship might range from 100 mL to several hundred milliliters. On the top of Chen and Cheng’s microwave (Chen and Cheng 2011), a 650 mL vessel with a 318 mm length, a connected nitrogen bottle, gauges, and thermometers are installed.

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Aside from glass jars and stainless-steel tanks with temperature and pressure sensors, a microwave input control system and a mechanical stirrer are also used.

3.2.1.3

Expansion and Extrusion

When materials travel through a die with a defined cross-section, they emerge with a fixed, definite profile. Sugar is extracted from biomass using the extrusion method. Mechanical extrusion pretreatment has several advantages: adaptability to changes, no degradation products, a regulated environment, and high throughput. Extruders are divided into two types: single screw and twin screw.

3.2.2

Physicochemical Pretreatment

Physicochemical approaches are utilized to dissolve lignocellulosic parts of the structure based on moisture levels, allowing the lignocellulosic content to be conveniently available for the hydrolysis step while preventing inhibitor development. Although these approaches are more difficult to apply, their significant impact on lignocellulose substrate pretreatment assures high production in the following bioprocesses (Theuretzbacher et al. 2015; Kumar et al. 2020). Physical pretreatment, in general, necessitates more expenditure of energy, making it a costly procedure that is not very economical on an industrial level. As a result, the focus of many previous studies is on process optimization and the criteria for lowering the energy, time, and costs of this treatment.

3.2.2.1

Explosion

High-pressure Steam Explosion (HPSE) This approach involves treating lignocellulosic biomass with high-pressure saturated steam and then rapidly lowering the pressure, resulting in explosive decompression of the biomass. The temperature of the steam explosion is set to 160–260  C, and the pressure is set to 0.69–4.83 MPa for a few seconds to minutes, after which the biomass of lignocellulosic plants is exposed and held at atmospheric pressure, triggering hemicellulose hydrolysis and ending the process with explosive decompression. The opportunity for cellulose degradation grows as cellulose breaks and lignin converts due to the high temperatures. Acid and many other acids generated throughout the steam explosion pretreatment played a part in the breakdown of hemicellulose. Because of turbulent material flow and rapid material flashing to air pressure, lignocellulosic material fragmentation occurs (Jørgensen et al. 2007; Li et al. 2007). H2SO4 or CO2 is utilized in steam explosion pretreatment to reduce time and temperature and inhibit product generation while increasing hydrolysis

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efficiency, leading to 100% hemicellulose removal. For softwoods, steam explosion pretreatment is ineffective; nevertheless, adding an acid catalyst throughout the procedure is required for the hydrolysis of the substrate through the enzyme. Biomass can be treated by steam at a specific temperature without the need for extensive dilution. At the end of the process, a sudden pressure release quenches the entire process and decreases the temperature. Rapid heat expansion opens the particle structure of biomass, which is employed to stop the reaction: moisture content, residence duration, size of the chip, and temperature all impact the steam explosion. Optimal hemicellulose hydrolysis and solubilization can be done in two ways: Low temperature and lengthy residence time vs high temperature and short residence time. Steam explosion pretreatment has an intense energy need. However, mechanical pretreatment requires 70% more energy than steam explosion pretreatment to achieve a similar reduction in particle size. So far, steam explosion pretreatment with the inclusion of a catalyst has been tried, and due to its costeffectiveness, it has the best chance of being scaled up at a commercial level. Steam explosion pretreatment is being employed on a small scale at the Iogen demonstration plant in Canada. Steam explosion pretreatment is an effective technique for hardwood and agricultural wastes.

Ammonia Fiber Explosion (AFEX) The AFEX technique is a physicochemical pretreatment technique. This low-temperature technique uses concentrated ammonia (0.3–2 kg ammonia/kg dry weight) as a catalyst. In a high-pressure reactor, ammonia is introduced to biomass; pressure is swiftly removed after 5–45 min of cooking. This operation is usually carried out at a temperature of roughly 90  C. Because of its volatility, ammonia can be collected and reused. AFEX works on the same idea as a steam explosion. Apparatus such as a reactor, thermocouple well, pressure gauge, pressure relief valve, needle valve, sample cylinder, temperature monitor, and vent are used in AFEX procedures. AFEX pretreatment of several blades of grass and herbaceous crops appears to boost the fermentation rate. AFEX technology is used to treat alfalfa, wheat chaff, and wheat straw. AFEX technique cannot remove hemicellulose or lignin. Hence, only a tiny amount of material is solubilized. During AFEX pretreatment, hemicellulose is breakdown into oligomeric sugars, and deacetylation occurs, resulting in hemicellulose insolubility. Ninety percent of cellulose and hemicellulose hydrolysis was achieved after AFEX pretreatment of Bermuda grass and bagasse. The efficiency of AFEX pretreatment diminishes as the lignin amount of biomass, such as newsprint, woods, nutshells, and aspen chips, increases. The maximal hydrolysis yield for newspaper and aspen chips after AFEX pretreatment was 40–50%. As a result, AFEX pretreatment is not recommended for the processing of biomass with high lignin content.

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Carbon Dioxide Explosion The treatment of supercritical CO2 explosions is classified as physicochemical pretreatment. Scientists attempted to generate a technique that would be less expensive than ammonia fiber explosion and work at temperatures lower than stream explosion temperatures. Supercritical carbon dioxide, which acts as a solvent, is used in this method. At ambient temperature, supercritical fluids are compressed above their critical point. Carbonic acid is formed when CO2 is dissolved in water, with unique properties that make it less corrosive. Carbon dioxide molecules enter tiny pores in lignocellulosic biomass throughout the process because of their small size. The CO2 pretreatment was carried out at a low temperature, which helped prevent sugar degradation by acid. When CO2 pressure is released, the cellulosic structure is broken, making the substrate more accessible to cellulolytic enzymes for hydrolysis (Haghighi et al. 2013; Kumar et al. 2009; Kumar et al. 2009; Kumar and Sharma 2017). Dale and Moreira (1982) pretreated alfalfa with carbon dioxide and detected a 75% theoretical release of glucose. Zheng et al. (1998) conducted studies on recycled paper and sugarcane bags to compare the effects of ammonia explosion, steam pretreatment, and CO2 pretreatment. CO2 explosion prevention is more economical than AFEX, according to the findings.

3.2.2.2

Mechanical/Alkaline Pretreatment

Individually applied mechanical techniques are rarely appropriate (Souza-Corrêa et al. 2013). Grinding-related mechanical pretreatment has the merit of being environmentally friendly, as it does not require the use of chemicals like alkalis or acids. However, because it is an energy-intensive operation, it is best utilized with different pretreatment procedures to save energy and reduce operational expenses. Lignin creates a physical and chemical hindrance to enzyme exposure to cellulose, and mechanical therapies cannot remove it (Miura et al. 2012; Shi et al. 2015). Various kinds of ball milling and compression milling (dry/wet, centrifugal, jet, and vibratory) have been utilized in a range of research studies, either alone or in combination with other procedures (acid, enzyme, and irradiation) to make material handling easier in future processing (Licari et al. 2016; Rahmati et al. 2020).

3.2.3

Chemical Pretreatment

To chemically pretreat lignocellulosic biomass, acids, alkalis, and organic solvents are utilized. One of the most promising is (Gupta and Verma 2015; Kumar et al. 2020), which can be highly effective in breaking down more complex-structured feedstocks (Pellera and Gidarakos 2018; Bhardwaj et al. 2020a). It also improves carbohydrate bioavailability by expelling lignin and/or decreasing cellulose

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crystallinity and degree of polymerization (Behera et al. 2014; Kumar and Verma 2020b). Chemical pretreatment is gaining popularity since it is usually less expensive and results in a faster and more effective breakdown of complex chemical molecules.

3.2.3.1

Liquid Hot Water

This treatment process is also referred to as “hot compressed water.” This type of pretreatment uses high temperatures (160–220  C) and pressures (up to 5 MPa) to keep water in a liquid state. Chemicals and catalysts, on the other hand, aren’t employed in the fluid hot water pretreatment procedure (Kumar and Sharma 2017). For around 15 min, liquid water is in contact with lignocellulosic biomass in this process. There is no need for quick leisure or extension in this pretreatment procedure because pressure prevents evaporation. This approach has proven to be extremely productive on sugarcane bagasse, wheat and rye straw, corncobs, and corn Stover. So many researchers utilize different terminology to describe this pretreatment process, such as solvolysis, aqueous fractionation, and hydro thermolysis (Kumar and Sharma 2017; Sparling et al. 2011; Zhuang et al. 2016). Depending on the direction of the biomass flow, pretreatment of liquid hot water can be done in three different manners, with the water flowing straight into the reactor. The first approach includes bringing the biomass slurry and water to a high temperature and keeping it there at pretreatment conditions for a certain period, and then cooling it down. Countercurrent pretreatment is the second approach, which involves pumping hot water against the current. The flow-through pretreatment method, which includes running hot water over lignocellulosic biomass that serves as a fixed bed, is the third alternative. Abdullah et al. (2014) showed the impact of liquid hot water pretreatment on the varied hydrolysis rates of cellulose and hemicellulose. The optimization of different parameters was examined in two steps of the process. The first phase, hydrolyzing hemicellulose, was carried out with reduced severity, but the second step, cellulose depolymerization, was done with great seriousness. The disadvantage of using liquid hot water for pretreatment is that it uses a lot of energy. The benefit of this method is not using chemicals or catalysts and inhibitors (Sparling et al. 2011).

3.2.3.2

Acid (Weak and Strong Acid Hydrolysis)

In this treatment, acids are applied to pretreat lignocellulosic biomass. Acid pretreatment releases inhibitory compounds, making it a less desirable pretreatment option. After acid pretreatment, inhibitory compounds such as furfurals, aldehydes, 5-hydroxymethylfurfural, and phenolic acids are generated in high numbers. Based on the type of final use, acid therapies are divided into two categories. The first treatment type is short (1–5 min) but uses a high temperature of >180  C, while the second treatment method is long (30–90 min) but uses a low-temperature 120  C.

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Because of the acid treatment, additional processes of biomass hydrolysis can be skipped although washing is required before the fermentation of sugars to eliminate the acid (Kumar and Sharma 2017; Sassner et al. 2008). Acid pretreatment necessitates the use of reactors that are resistant to corrosive, hazardous, and toxic acids; as a result, acid pretreatment is quite costly. Rectors have been produced in various ways, including flow-through, percolation, shrinking-bed, countercurrent rector, batch, and plug flow. The ability to recover concentrated acid at the end of the treatment is crucial for improving the economic viability of acid pretreatment. They concentrated on lignocellulosic biomass treatment.

Sulfuric Acid The most often used acid is sulfuric acid. Acid pretreatment can improve the hydrolysis process for producing fermentable sugars from lignocellulosic biomass. Poplar, switchgrass, spruce, and corn stover are all commonly pretreated with sulfuric acid. The sugar content of Bermuda grass and rye was reduced by 19.71% and 22.93%, respectively, after acid pretreatment. Rice straw was processed with aqueous ammonia and dilute sulfuric acid in two phases in a percolation reactor. When using ammonia, a 96.9% reducing sugar yield was attained; however, when using diluted acid, a 90.8% yield was achieved. Eulaliopsis binata released 21.02% of sugars, 3.22% lignin, and 3.34% acetic acid, as well as inhibitors in trace levels when exposed to dilute sulfuric acid (Kim et al. 2011; Tang et al. 2013). Because it is less expensive and more effective, pretreatment with a sulfuric acid concentration of 4 wt% is preferred. The dilute H2SO4 responsible for biomass breakdown causes xylose to be broken down further into furfural. At high temperatures, dilute H2SO4 hydrolysis is most efficient (Mosier et al. 2005). Hemicellulose must be eliminated to improve glucose output from cellulose, and diluting H2SO4 is an essential instrument for this. For cost-effective biomass conversion, a high xylan-to-xylose ratio is mandatory. Xylan makes about one-third of the total carbohydrate in most lignocellulosic materials. The first is characterized by intense temperature and constant flow method for modest solid loadings, whereas the second is attributed to the standard batch procedure for substantial solid loadings. The first type’s temperature and solid loadings are >160  C and 5–10%, respectively, whereas the second type’s temperature and solid loadings are around 160  C and 10–40%, respectively (Brennan et al. 1986; Esteghlalian et al. 1997). In addition to sulfuric and hydrochloric acids, other acids such as oxalic and maleic acids are applied to pretreat lignocellulosic biomass. Oxalic and maleic acids have a higher pKa value and solution pH than H2SO4. Since dicarboxylic acids have two pKa values, they are more efficient at hydrolyzing biomass as sulfuric and hydrochloric acids. Lower yeast toxicity, no odor, a more comprehensive pH and temperature range for hydrolysis, and no inhibition of glycolysis are other advantages. The khyd/kdeg ratio of maleic acid is high, favoring cellulose hydrolysis over glucose breakdown.

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(Lee and Jefferies 2011), the impacts of oxalic, sulfuric, and maleic acid preprocessing on feedstock were investigated when the severity factor was combined (CSF). Maleic acid produces a more significant xylose and glucose amount that are higher than oxalic acid levels.

Phosphoric Acid Another study by Geddes et al. (2010) showed the production of sugar monomers by sugarcane bagasse and found that pretreatment with 1% (w/w) H3PO4 at 160  C for 10 min yielded greater monomers, which this pretreatment method was reportedly proper because it successfully hydrolyzes hemicellulose in the substrate. As a result, cellulose is more easily digestible by enzymes.

3.2.3.3

Alkaline Hydrolysis

The hemicelluloses ratio hydrolyzes under low pH, but the cellulose and lignin parts do not. Instead, at a higher pH, lignin is solubilized, which is the basis for several pulping processes used to produce high-quality journal papers. Depending on the degree of the pretreatment, the amount of solid material that dissolves varies substantially. Hemicelluloses and cellulose can be transformed to oligo- or monosaccharides when a high acid level is applied at high temperatures. In considerably more acidic circumstances, carbohydrates break down to different compounds such as furfural, 5-hydroxymethylfurfural (HMF), levulinic acid, and so on. Temperatures of 140–200  C are typical, with a minute to hours of residence time (Galbe and Wallberg 2019). Some bases, in addition to acids, are utilized in biomass processing. The amount of lignin in the sample had a significant impact on the alkaline treatment’s outcome. Although it takes days and hours, treatment with alkali necessitates minimal pressure, temperature, and environmental conditions than other pretreatment processes. Sugar decomposition is reduced in alkali treatment compared to acid treatment, and caustic salt removal and recovery are also possible and straightforward. Ammonium, sodium, calcium, and potassium hydroxides are all alkali pretreatment agents; nevertheless, the much more commonly utilized substance is sodium hydroxide alkali pretreatment agent, while the least is calcium hydroxide but the most influential of all alkali pretreatment agents. Calcium can be bounced back in insoluble calcium carbonate by neutralizing it with CO2. Limekiln technology can be used to renew calcium hydroxide. A temperature controller, a tank, a CO2 scrubber, a water jacket, a water and air manifold, a pump, a tray, a frame, a temperature sensor, and a heating element are required for alkali pretreatment. Making a lime slurry utilizing water is the initial step in pretreatment. The next stage is to spray the slurries lime over the biomass and then keep it for hours or days, depending on the circumstances. Increasing temperature lowered contact time, according to Elshafei et al. (1991), Kim and Holtzapple (2006), Lee et al. (1994), and MacDonald et al. (1983). The crystallinity index

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rises as lignin and hemicellulose are eliminated during lime pretreatment. The structural properties that emerge from lime pretreatment influence the hydrolysis of pretreated biomass. Chang and Holtzapple (2000) discovered a connection between three structural aspects: lignin, acetyl content and crystallinity, and enzymatic digestibility. They concluded that (1) significant delignification is adequate to give high digestibility regardless of crystallinity or acetyl concentration. (2) Hydrolysis is aided by the removal of parallel barriers such as dignification and deacetylation. (3) While crystallinity has little impact on the final sugar yield, it is involved in the initial step of the hydrolysis process. Based on these principles, the lignin amount should be reduced to 10%, and it is necessary to terminate all acetyl groups through a particular pretreatment method. Consequently, alkaline pretreatment is critical for enzyme exposure to cellulose. Lignin elimination can significantly improve enzyme function by improving enzyme access to cellulose and hemicellulose while removing nonproductive adsorption sites.

Sodium Hydroxide In another work, Harun and Geok (2016) used NaOH to pretreat rice straws. At 55  C, the straw was prepared for 1 h and 3 h, respectively. All samples were treated with a 2–12% (w/v) NaOH solution and a 1:20 (w/v) rice straw to NaOH solution ratio. One of the outcomes was that using 1 h of 12% NaOH enhanced the glucan concentration by 85.6% (compared to untreated straw). In this setting, the delignification of the straw was likewise at its maximum, at 79.6%. Straw was treated for 3 h with 2% NaOH although it gave the highest effective total carbohydrate content, at 79.2% (Galbe and Wallberg 2019).

Hydrogen Peroxide H2O2 is possible to combine the alkaline reagent and hydrogen peroxide. In a broad range of lignocellulosic materials, H2O2 enhances enzymatic accessibility, resulting in higher enzymatic hydrolysis yields (Dutra et al. 2018). Lignocellulosic biomass is bleached with the bleaching chemical H2O2. Although many free radicals are created during pretreatment, the biomass does not leave residues since it dissolves into oxygen and water with few secondary products. The oxidative fragmentation of lignin side chains leads to lignin removal from the lignocellulosic matrix (Song et al. 2013). In terms of delignification and enzymatic digestibility, Yuan et al. (2018) discovered that alkaline hydrogen peroxide beats single alkali reagents. However, keeping a constant pH throughout the procedure for avoiding hemicellulose excision, which is an exceptionally essential criterion for process efficiency (Zhuang et al. 2016), limits this technique. Delignification using alkaline hydrogen peroxide enhances lignin depolymerization and requires gentle temperature and pressure settings (Su et al. 2015), lowering inhibitor generation (Rabelo et al. 2014).

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Ayeni et al. (2019) evaluated the operating settings for alkaline hydrogen peroxide pretreatment of sugarcane bagasse. They discovered that 0.3% hydrogen peroxide concentration and 100  C for 4.6 h were the optimal pretreatment conditions. Siciliano et al. (2016) found that treating olive mill residues anaerobically with a low oxidant concentration at room temperature increased the anaerobic treatability by up to 0.328 L/g of COD (chemical oxygen demand) removed. Katukuri et al. (2017) also discovered that treating Miscanthus floridulus substrate with 0.8% H2O2 boosted methane yield by 49% over untreated Miscanthus floridulus substrate. Alencar et al. (2017), on the other hand, investigated the H2O2 recycling technology. They looked at the ability of recycled H2O2 in five different reuse cycles and discovered that each one was less efficient than the last. Before anaerobic digestion, Dahunsi (2019b) investigated the impact of acid or alkaline H2O2on pineapple skip preprocessing. The temperature was adjusted around 75–115  C, the residence period was varied between 6 and 46 min, and the H2SO4 concentration was 0–2% (v/v) according to response surface methods. Alkaline H2O2 was used in a similar technique. The study’s primary findings were that enhanced alkali pretreatment resulted in lignin depletion and a biogas output that was 67% more than the treatment by acid (Galbe and Wallberg 2019).

Wet Oxidation In this pretreatment process, biomass is treated at high temperatures (>120  C) for half an hour at 0.5–2 MPa pressure using oxygen/air and water or hydrogen peroxide Haghighi et al. 2013; Varga et al. 2003). This kind of pretreatment is also employed for wastewater treatment and soil remediation. For pretreatment of lignin-rich biomass, this approach has proven to be quite effective. The efficiency of the wet oxidation preprocessing method is influenced by elements, for example, reaction time, oxygen pressure, and temperature. Water behaves like acid at high temperatures, inducing a hydrolysis reaction as the concentration of hydrogen ions grows with temperature, lowering the pH value. In wet oxidation pretreatment, pentose monomers are generated due to hemicellulose breakdown, and lignin is oxidized, but cellulose is unaffected. Some reports of alkaline peroxide or sodium carbonate are added to the mix. The presence of these chemical agents aids in lowering temperature reactions and reducing inhibitory compound production. Attempts to improve hemicellulose breakdown at high temperatures result in inhibitory chemicals such as furfural and furfuraldehyde. However, the number of inhibitors produced by wet oxidation pretreatment is far lower as compared to that produced by liquid hot water pretreatment or the steam explosion approach. Because of two factors, this technique has very little chance of being used on a large scale. The volatile nature of oxygen is one factor, while overprice of hydrogen peroxide utilized during the procedure is another (Bajpai 2016).

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Ozonation Pretreating lignocellulosic biomass with ozone is an ideal approach to reducing lignin. The application of ozone pretreatment to biomass improves its in vitro digestibility. This preparation does not produce inhibitors, a significant benefit because conventional chemical pretreatment leaves hazardous residues. Ozone (O3) is used as an oxidizer in ozone pretreatment that breaks lignin. O3 gas is a powerful oxidant that breaks down lignin and creates soluble compounds with lower molecular weight. Wheat straw, bagasse, cotton straw, green hay, poplar sawdust, peanut, and pine can all be prepared with ozone by the breakdown of lignin and hemicellulose; however, all of them are the only materials that can be O3 pretreated (Ben-Ghedalia and Miron 1981; Kumar et al. 2009; Quesada et al. 1999). A catalytic destroyer of O3, a trap made of iodine for measuring catalyst potential, an O2 cylinder, an O3 generator, a three-way valve, an O3 UV spectrophotometer, a valve for controlling pressure, and a gas used in the process are all included in an ozonolytic apparatus. Because lignin oxidation decreases as biomass moisture content rises, moisture amount substantially influences lignin oxidation via ozone exposure. At lower water concentrations, ozone mass transfer is reduced, which impacts its reactivity with biomass. Blocking pores by water film causes a more extended ozone residence period (Mamleeva et al. 2009). Organic acids production lowers the water pH during ozonolysis. Alkaline environments trigger delignification because lignins linked to carbohydrates are removed (Chen et al. 1984; Travaini et al. 2015). The formation of inhibitory chemicals is linked to biomass delignification. Delignification produces several aromatic and polyaromatic chemicals (Travaini et al. 2013). In a study, Bule et al. (2013) discovered structural alterations in lignin; NMR research demonstrated different lignin subunits have aromatic opening and degradation of -O-4 moieties. In a spectrum, the concentration of aromatic carbon signals dropped. Changes in methoxy groups were discovered, indicating that the ester-linked structure was disintegrating. Batch reactors, Drechsel trap reactors, fixed bed reactors, rotatory bed reactors, and multilayer fixed bed reactors are the reactor models applied for O3 biomass processing. Most experts employed plug flow reactors (Li et al. 2015). Heiske et al. (2013) investigated the properties of single-layered and multilayered bed reactors to increase the transformation of wheat straw to biogas. A single-layered reactor produced straw with a lignin content of 16.2%, while a multilayered reactor produced straw with a lignin content of 7.2% at the bottom layer. In ozone-pretreated wheat straw, Kádár et al. (2015) discovered that wax breakdown drives the formation of fatty acid molecules. When corn stover was processed with ozonolysis, Williams (2006) found that over 49% of the lignin was broken down.

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Solvent Extraction

Scientists have been interested in the application of ionic liquids to pretreat lignocellulose over the term. Ionic liquids containing cations or anions are a new type of solvents with great thermal stability and polarity, as well as a lower melting point and vapor pressure (Behera et al. 2014; Zavrel et al. 2009). Significant organic cations and modest inorganic anions make up ionic liquids: temperature, cations and anions, and pretreatment time all impact the interaction of ionic solvents with biomass. Ionic liquids conflict with lignocellulosic materials for h-bonding, disrupting networks.1-ethyl-3-methylimidazolium diethyl phosphate-acetate, 1-butyl-3methylimidazolium-acetate, cholinium amino acids, cholinium acetate, 1-allyl-3 methylimidazolium chloride, and chloride are ionic liquids used to treat rice husk, water hyacinth, rice straw, kenaf powder, poplar wood, wheat straw, and pine (Kumar and Sharma 2017). Imidazolium salts are one of the commonly applied ionic liquids. Dadi et al. (2006) found that using 1-butyl-3-methylimidazolium chloride as a pretreatment doubled the output and frequency of hydrolysis. Liu and Chen (2006) employed 1-butyl-3-methylimidazolium chloride, referred to as (Bmim-Cl), to pretreat rice straw and observed that it modified the wheat straw structure by Bmim-Cl, resulting in a considerable improvement in the hydrolysis process. Polymerization and crystallinity were reduced, thanks to Bmim-Cl. Research by Kuo and Lee (2009) found a twofold increase in hydrolysis yield from sugarcane bagasse compared to untreated bagasse. In a study by Li et al. (2010), 1-ethyl-3-methylimidazolium-acetate was used to pretreat switchgrass at a temperature of 160  C for 3 h to eliminate lignin. Tan et al. (2011) observed that 1-butyl-3-methylimidazolium chloride pretreatment of palm trees resulted in 62.9% lignin reduction, enzymatic digestion was improved, while cellulose crystallinity was minimized. After ionic liquid pretreatment, minor changes in biomass composition occurred, but considerable alterations in biomass structure were found. Ionic liquid pretreatment is less desirable than alternative treatments due to its chemical and thermal durability, least hazardous processing circumstances, the low vapor pressure of solvents, and the ability to establish a liquid condition throughout a broad range of temperatures. Ionic liquids are simple to recycle and don’t degrade. Cellulase unfurls and inactivates due to the incompatibility between cellulose with ionic liquids, which is a downside of using ionic liquid pretreatment. Since cellulose dissolves at less temperatures and viscosities, viscosity is an essential factor in terms of average energy consumption, ionic liquids should be addressed. Mäki-Arvela et al. (2010) proposed that high temperatures result in different side reactions and unfavorable effects, such as ionic liquid stability deterioration.

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Biologicals Pretreatments

Pretreatment with biologicals is a viable alternative to chemical pretreatment, but it has a slower reaction rate which is inconvenient for business (Wyman et al. 2018).

3.2.4.1

Fungi

Various microorganisms, including white, brown, and soft rot fungus, are used to pretreat lignocellulose. White-rot fungi are the most well-known and influential delignification microorganisms (Raveendran et al. 2016; Agrawal and Verma 2021). Although brown rot fungi contribute to lignin degradation, white-rot fungi are the most prominent and influential delignification microorganisms. Auxiliary enzymes such as hydrolytic enzymes, cellobiose dehydrogenase, aryl alcohol oxidase, glyoxal oxidase, copper oxidase, and cellobiose dehydrogenase are released by most fungi that degrade lignocellulose, causing the concurrently or specific destruction of cellulose and hemicellulose, as well as lignin (Rudakiya and Gupte 2017; Alam et al. 2021; Verma 2022; Bhardwaj et al. 2020b). These enzymes help break down lignin and polysaccharides by producing free radicals and intermediates. The effects of biological pretreatment with laccase, manganese peroxidase, and versatile peroxidase at different incubation times—0, 6, and 24 h—were investigated by Schroyen et al. (2014). The presence of phenolic compounds, which are methane inhibitors, was not detected in the pretreatment. The laccase enzyme increased biomethane synthesis by 25% after 1 day of the process of incubating bacteria, but prearrangement with peroxidase enzymes increased the biomethane output by 17% while decreasing the process of incubating bacteria incubation time from 1 day to 6 h. To improve methane generation, Liu et al. (2017) used Ceriporiopsis subvermispora to evaluate fungal pretreatment of two forest wastes, namely hazel and acacia branches, and two agricultural leftovers, namely barley straw and sugarcane bagasse. Compared to untreated hazel, the fungus’ manganese peroxidase and laccase increased methane emissions by 100%. While fungal pretreatment of lignocellulosic biomass is favorable to the environment and the economy, they are timeconsuming. According to Liu et al. (2017), a unique bioreactor should develop aerobic and aseptic conditions for fungal pretreatment. According to Van Kuijk et al. (2015), selecting substrate-specific and effective strains and recovery duration and carbohydrate losses can be reduced by changing the cultural conditions. Biogas generation from lignocellulosic biomass can be boosted via inoculation with Neocallimastix frontalis, an anaerobic fungus isolated from the rumen, according to Dollhofer et al. (2018).

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3.2.4.2

71

Bacteria

Microbial pretreatment surpasses enzymatic pretreatment in the anaerobic digestion process due to their greater variety and tolerance to external parameters such as temperature and pH. Rouches et al. (2018) isolated bacterial strains from soil (Bordetella muralis VKVVG5), silverfish stomach (Citrobacter werkmanii VKVVG4), and millipede (Paenibacillus sp. VKVVG1) and used them to speed up the hydrolysis of water hyacinth. Water hyacinth was pretreated for 4 days with Citrobacter werkmanii VKVVG4 at a rate of 109 CFU/mL, which enhanced the solvability by 33.3%. Others, on either hand, proposed using anaerobic lignocellulose degrading consortiums. According to Kong et al. (2018), in mesophilic and thermophilic settings, anaerobic consortia TC-5 decomposed wheat straw by 45.7% and increased methane output by 22.2% and 36.6%, respectively. During 24 h of exposure, rumen fluid was proven to enhance the productivity of rapeseed stems and leaves by 47% (Baba et al. 2017). Yuan et al. (2016) pretreated cotton stalks with the thermophilic microbial consortia MC1. The findings confirmed that the amounts of soluble chemical oxygen and volatile organic compounds rise rapidly in the early phases of the pretreatment, peaking the highest methane CH4 output at 128 mL/g over 6 days of MC1 pretreatment. The CH4 emission was not distinguishable between 122 mL/g and 122 mL/g. Sawdust inoculated with a microbial consortia LCDC obtained from decaying sawdust produced 72.6% more methane than control, according to Ali et al. (2017). They found a significant decline in lignocellulose contents during 10 days of treatment (Hernández-Beltrán et al. 2019).

3.3

Conclusions

The most prevalent materials for biofuel production are lignocellulosic substrates, and this digestion procedure aids in waste disposal while also providing one of the most important sources of renewable bioenergy. Because of their low bioactivity, the structure of lignocellulosic residues continues to pose technological challenges, and proper implementation of digesting technology requires the pretreatment of these resistant substrates. However, as it can be observed, the key constraints are energy expenses, operating expenses, and the generation of anti-proliferative chemicals, all of which substantially impact the downstream bioprocess for producing biofuel or additional products with enhanced value. A mix of diverse pretreatment may provide a solution that can be tailored to the substrates and downstream bioprocesses to yield bioenergy and certain other products. Biorefineries that produce two or more bioproducts on the same system are inclusive biorefineries, which could be a potential approach for establishing the techno-economic viability of lignocellulose substrate use.

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Competing Interests All the authors declare that they have no competing interests.

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Chapter 4

The Pioneering Role of Enzymes in the Valorization of Waste: An Insight into the Mechanism of Action Anupama Binoy , Revathy Sahadevan and Sushabhan Sadhukhan

, Suchi Chaturvedi,

Abstract Finite petro-based reserves and a surge in environmental pollution demands the valorization of waste into revenue streams like biofuels and other industrial commodities. Enzymatic technology provides an eco-friendly platform for the same with higher product yields. Enzymes act as a catalyst in the reaction, and the matter of value addition in this technology is its requirement in low quantity and reusability. They have been included in the valorization of lignocellulosic (woody, agro, and food) waste, treatment of wastewater, and degradation of non-biodegradable hazardous waste. Microbial flora has enormously experimented as well as explored in the conversion of this waste into valuable products. In addition to that, protein engineering and metabolic engineering have been seen as new biotechnological trends in the same field. In this chapter, we will focus on different classes of hydrolytic enzymes based on the structural composition of different types of biomass with special attention to their catalytic activity. The mechanistic action of these enzymes will also be discussed in lieu of their use at various stages in the

Revathy Sahadevan and Suchi Chaturvedi contributed equally to this work. A. Binoy · R. Sahadevan Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad, Kerala, India e-mail: [email protected]; [email protected] S. Chaturvedi Department of Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Indore, Madhya Pradesh, India e-mail: [email protected] S. Sadhukhan (*) Department of Chemistry, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Department of Biological Sciences and Engineering, Indian Institute of Technology Palakkad, Palakkad, Kerala, India Physical & Chemical Biology Laboratory, Indian Institute of Technology Palakkad, Palakkad, Kerala, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_4

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transformation of waste to value-added substances. We will also shed light on the future advancement through the biotechnological revolution in the field of enzyme technology. Keywords Hydrolytic enzymes · Valorization of waste · Lignocellulosic waste · Wastewater · Biorefinery

Abbreviations AAO BOD CAZy CLEAs COD DNA DyP EG FAD GH GHG GLOX GOS LiP MnP MSW P2O PAH PCR PE PE PET PG PGL PL PMG PMSF POPs PP PS PUs PVC RNA TPA VP XYNII

Aryl alcohol oxidase Biological oxygen demand Carbohydrate-active enzyme Cross-linked enzyme aggregates Chemical oxygen demand Deoxyribonucleic acid Dye-degrading peroxidases Ethylene glycol Flavin Adenine Dinucleotide Glycoside hydrolases Greenhouse gas emission Glyoxal oxidase Galacto-oligosaccharides Lignin peroxidase Manganese peroxidase Municipality solid waste Pyranose 2-oxidase Polycyclic aromatic hydrocarbons Polymerase chain reaction Pectin esterase Polyethylene Poly ethylene terephthalate Polygalacturonase Polygalacturonate Pectin lyase Polymethygalacturonase Phenyl methane sulfonyl fluoride Persistent environment pollutants Polypropylene Polystyrene Polyurethane Polyvinyl chloride Ribonucleic acid Terephthalate Versatile peroxidases; Endo-1,4-ß-xylanase II

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Introduction

The global surge in population and simultaneous automation have resulted not only in depletion of petro fuels reserve but have also piled up different forms of waste globally. In the global waste management outlook, GWMO 2015 stated that the global waste piling accounts for nearly 7–10 billion tonnes which include households, commercial, industrial, and construction-based waste (Wilson and Velis 2015; Agrawal and Verma 2022). Moreover, the major issues in front of humankind have been climate change, greenhouse gas emissions (GHG), depleting resources, and increasing pollution for a long. Waste management has existed in our society for a long in the form of landfills, incineration, composting, etc. but they are least suitable for organic waste due to problems associated with them like the generation of toxic gases such as methane and toxic leachates to the underground water bodies, etc. This calls upon the urgency to frame an economy based on renewable resources fulfilling the shooting energy demands (Kuo 2019; Goswami et al. 2022; Agrawal and Verma 2021) and also builds up sustainable methods to convert our waste into valuable revenue streams. Adapting to a sustainable way can be envisioned by valorizing waste into biofuels (different forms of bioenergy to replace fossil fuels), biomaterials, and other value-added bio-ingredients. This process is commonly associated with the concept of biorefineries where the waste biomass is upgraded and transformed into a spectrum of invaluable and marketable commodities. Waste can be of different forms. The most common and important problem faced during the valorization of waste is the complexity of the composition of waste. Organic or biomass waste is solid or liquid waste that can be found in many forms like agro and food waste, forestry residues, waste generated from food processing industries, etc. The basic units of the biomass waste are rich in protein, sugar, and fat, which indeed make them an ideal feedstock for enzymatic valorization. Enzymes are introduced at various steps in the process of valorization. For instance, the lignocellulosic mass from the agro-waste is subjected to pretreatment by exposing them to delignifying enzymes which removes the protective lignin component. Thus, enabling the hydrolyzing enzymes to easily solubilize the polymers like cellulose, and hemicellulose into their monosaccharides and oligosaccharides. These monomer units are either subjected to fermentation using microbes for the production of biofuels or enzymatic modification like oxidation/phosphorylation for valorizing into valued products (Andler and Goddard 2018; Bhardwaj and Verma 2021). Enzymes are molecular catalysts triggering biochemical reactions. They catalyze the reaction with high substrate and product selectivity at optimum temperature and pressure. Reusing enzymes for several reactions further makes the procedure cost-effective. All this opens up an opportunity for introducing greener processing strategies that are more sustainable for the ecosystem (Kennedy et al. 2006). Enzymes are ubiquitous in all forms of life performing different purposes and therefore can be extracted from them through purification and characterization (Yada 2015; Bhardwaj et al. 2021a; Kumar and Verma 2020a). Systems biology has encompassed a role in understanding the molecular basis of different enzymes. Through various bioinformatics tools and

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algorithms, system biology enables us to form a link between different biological components in an organism thereby allowing us to modify and decipher newer enzyme candidates involved in waste valorization as well (Bhatt et al. 2019). Here in this chapter, we will be focusing on different enzymes which participate in hydrolysis of biomass component. As enzymes are very specific to the composition of its substrate, we will also shed light on the composition of different types of biomasses. To better understand the action of hydrolyzing enzymes and their application, it is necessary to take a glimpse at the different characteristics of enzymes and the different factors which affect the activity of enzymes. This chapter will also highlight the advanced enzyme technology and trending opinion on increasing the efficacy of enzymes for valorization of waste.

4.2

Enzymes and Their Characteristics

Enzymes are biological catalysts known to increase the rate of biochemical processes happening inside a living organism (Blanco and Blanco 2017). They were first introduced by Frederick W. Kuhne as molecules of higher molecular weight that help in the fermentation of sugar to alcohol. Enzymes are mostly composed of proteins except for some RNA which exhibit enzymatic characteristics in certain biological processes. Enzymes are associated with cofactors (inorganic metal ions like Cu2+, Zn2+, Mn2+) and/or coenzymes (organic or organometallic complexes) for their activation and function (Renneberg et al. 2017). An enzyme without the cofactor or coenzyme is known as an apoenzyme and the complete active form is referred to as the holoenzyme (Litwack 2018). The accurate structural conformation of enzymes plays a crucial role in determining their activity and any parameters, physical or chemical, that alter the native confirmation will affect the efficiency of their catalytic activity. Based on the type of catalytic reactions, enzymes are categorized into six groups. Oxidoreductases catalyze oxidations or reduction reactions. Transferases help in the transferring of functional groups between two molecules. Hydrolases catalyze hydrolysis (lysis in the presence of water). Lyases are involved in the removal or addition of groups to form or reduce double bonds. Isomerases catalyze the internal arrangement of atoms in a molecule to form isomers and finally, lipases catalyze condensation reactions to form bonds between carbon–sulfur, carbon–carbon, or carbon–nitrogen (Blanco and Blanco 2017; Kumar and Verma 2020b).

4.2.1

Enzyme–Substrate Interaction

An enzyme facilitates a suitable environment for the substrates to form products at an enhanced rate. The substrate undergoes non-covalent interaction with the enzyme in a specific site called the active or substrate-binding site of the enzyme to form a

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substrate–enzyme complex. This enzyme–substrate complex later transforms into a product and releases the enzyme back. The catalytic action of enzymes is favored at optimal pH, temperature and ionic strength, etc. and any alterations will affect the catalytic activity of the enzymes. For example, a temperature rise will cause an alteration in the enzyme’s native conformation (denaturation) which will reduce their catalytic activity. Small molecules can also affect the activity of enzymes and are known as inhibitors. The important difference between competitive and noncompetitive inhibitors is their preference for the binding site on enzymes. Competitive inhibitors compete for the active site on the enzyme whereas noncompetitive inhibitors engage with sites other than that of the active site. Both the inhibitors eventually bring conformational changes in the structure of the enzyme and facilitate inhibition of its activity. The active site of an enzyme has a unique and specific sequence of amino acids which in turns increases substrate specificity and selectivity. Enzyme specificity is first explained by the “Lock and Key model” introduced by Emil Fischer in 1894. The assumption of the rigid structure of enzymes was falsified later in 1958 through the “Induced Fit Model” proposed by Daniel Koshland where the concept of the transition state of the enzyme–substrate complex was introduced. According to this model, the conformation of the enzyme’s active site as well as in some cases conformation of the substrate itself will undergo small changes during the formation of the enzyme–substrate complex (Blanco and Blanco 2017).

4.2.2

Enzyme Thermodynamics and Kinetics

An enzyme, just like any catalyst increases the reaction rate by lowering the activation energy of the reaction (Fig. 4.1). The kinetics of the enzyme–substrate is explained by Leonor Michaelis and Maud Menten in 1913. Michaelis-Menten theory is based on the following reaction between enzyme (E) and substrate (S) to yield the product (P) through the formation of the enzyme–substrate complex (ES).

Fig. 4.1 Energy profile diagram of an enzymecatalyzed and uncatalyzed reaction. Where S represents a substrate, ES is the enzyme–substrate complex, EP is the complex of enzyme and product just before their degradation to product, P, and # represents the transition state

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Fig. 4.2 The graphical representation of (a) Michaelis-Menten equation; (b) Lineweaver-Burk plot in the absence of inhibitor and the presence of competitive and non-competitive inhibitors k1

k2

E þ S Ð ½ES ! P þ E k 1

ð4:1Þ

Michaelis and Menten derived an equation for the rate of the catalytic action of enzymes based on the assumption that the formation of ES and the reverse reaction to E and S is faster than its conversion to P and E and then applied the steady-state approximation for ES complex. The Michaelis-Menten equation is given as, V0 ¼

V m ½ S ½ S þ K m

ð4:2Þ

where V0 is the rate at which ES converts into a product, Vm is the maximum reaction rate at the saturated concentration of the substrate, [S] is the concentration of substrate, and Km is Michaelis constant, it is the concentration of the substrate needed to acquire Vm/2. The graphical representation of this equation, Vo vs [S] plot is given in Fig 4.2a. This equation is applicable to single enzyme–substrate interaction at constant enzyme concentration. At a low concentration of substrate, the enzymatic reaction follows first-order kinetics where the rate of reaction is proportional to substrate concentration. At a high concentration of substrate, the reaction follows zero-order kinetics, which means the rate of reaction is independent of substrate concentration. Enzyme activity in the presence of inhibitors is well understood from the Lineweaver-Burk plot which is a graphical representation of the reciprocal of the Michaelis-Menten equation, as shown in Fig 4.2b.   1 1 1 Km ¼ þ V max ½S V 0 V max

ð4:3Þ

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Chemical and Structural Factors Guiding Enzyme Hydrolysis

Biomass waste is biodegradable organic matter produced directly or indirectly by the energy utilized from the sun through the process of photosynthesis. The source of waste is very diverse and varied in terms of its composition. They are mainly composed of constituents like lignocellulosic biomass, starch, chitin, triglycerides, proteins, etc. (Tuck et al. 2012; Kumar and Verma 2020b). Due to the high fixed carbon content in them, these feedstocks have been in past and are even now used to generate energy and heat by burning them which could pose threat to the environment by the release of toxic air pollutants such as PAH (polycyclic aromatic hydrocarbons), particulate matters, POPs (persistent organic pollutant) (Sivertsen 2006). Policymakers have directed the industries to focus on the sustainable valorization of worldwide generated biomass so that such a resourceful matter is not just dumped as waste or burned into ashes. Currently, they are utilized as major feedstock in biorefineries for biofuels production, biogas, and biofertilizers.

4.3.1

Composition of Lignocellulosic Biomass

Lignocellulosic biomass is categorized based on the source of its generation. It could be organic matter from woody terrestrial forest residues, herbaceous residues from agriculture (corn cobs, sugarcane bagasse, rice and wheat husk, fruits and vegetable residues generated from fields and market), green waste from Municipality Solid Waste (MSW), animal and human sources, aquatic organic mass, and organic mass generated by anthropogenic ways as well. The sustainable utilization of biomass offers a huge advantage as they are widely available worldwide, reduces the overall cost of fuels by introducing them as an alternative source, and finally, contributes to the reduction of greenhouse gas emissions. Moreover, the overall production and process cycle of sustainable alternatives exhibit a zero-carbon dioxide balance. Lignocellulosic biomass is structurally composed of cellulose, hemicellulose, protein, lipids, etc. They also contain some active ingredients like antioxidants, polyphenols, lignin, pigments/carotenoids, etc. These constituents are arranged in layers to form the complete lignocellulosic biomass structure with lignin being the outermost layer, hemicellulose occupying the middle space, and cellulose placed at the core of the mass (Fig. 4.3). Interestingly, cellulose is the major substrate in the biorefineries for biofuels and chemical commodities production. The percentage of these components varies based on the type and source of lignocellulosic biomass. Almost 15% of the total lignocellulosic mass comprises protein also. The lignin component renders a high resistance to cellulose access by the hydrolytic enzymes. Various pretreatment methods have been adopted to remove the lignin part to give more access to hydrolytic enzymes to degrade the high polysaccharide component of cellulose and

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Fig. 4.3 Structural representation of lignocellulosic biomass: The plant cell wall is composed of a lignocellulosic structure where lignin forms the outermost covering, followed by hemicellulose and the most internal part is made up of cellulose components

hemicellulose. Depending upon the chemistry of the components present in the lignocellulosic biomass, enzymes are selected to degrade it.

4.3.1.1

Cellulose

Cellulose is an important cell wall component of the plant. It is a high molecular weight polysaccharide composed of D-glucose units linked with each other through β-1,4-glycosidic bonds forming the basic repeating unit called cellobiose, 4-O-β-Dglucopyranosyl-D-glucopyranose (structural basis of cellulose) (Fig. 4.4a). A cellulose primary chain is composed of almost 500–1400 monomer units. These primary chains are further arranged in a parallel array to form the higher structural unit called microfibril. Several microfibril units form cellulose fibril, the higher structural unit (Robak and Balcerek 2018; Bhardwaj et al. 2021b). The extensive intramolecular (O6–O2 and O3–O5) and intermolecular (O3–O6) hydrogen bonds and Van der Waals forces in cellulose structures give its crystalline nature, high tensile strength, and recalcitrance to hydrolysis. The amorphous cellulose corresponds to regions where the above-mentioned molecular bonds are disrupted giving twists and torsions to the structure resulting in interspersed disordered regions in cellulose. The structural or crystalline region of cellulose is highly packed not allowing even a single water

The Pioneering Role of Enzymes in the Valorization of Waste: An. . .

Fig. 4.4 Chemical structures of components present in lignocellulosic plant’s cell wall: (a) basic skeletal structure of cellulose: Glucose units combined by β(1,4)-glycosidic linkages; (b) basic skeletal structure of hemicellulose having β-(1,4)-xylose-β-(1,4)-agarose-β-(1,4)-mannose-β-(1,4)-glucose-α-(1,3)-galactose motifs; (c) basic skeletal structure of lignin component; and (d) Structure of lignin in poplar plant. Where S is the syringyl group, G is the guaiacyl, and SP is the synapyl basic groups

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molecule to enter, but the distorted amorphous region is very easily accessible to enzyme hydrolysis (Betts 1991).

4.3.1.2

Hemicellulose

Hemicellulose stands second to cellulose in terms of its abundance on the planet. In comparison to cellulose, hemicellulose is highly branched with almost 500–3000 sugar units as monomers. Hemicellulose is a heteropolymer comprising of side chains with xylans as pentoses, mannans, and glucomannans as hexoses or arabinogalactans and galactans as galactose units. Apart from monosaccharides, they also accompany typical uronic acids like D-glucuronic, 4-O-methylglucuronic, and D-galacturonic acids (Fig. 4.4b). Xylan: Xylans are water-soluble polysaccharides made up of repeating units of βD-xylopyranose linked by β-(1,4)-glycosidic bonds. This primary chain also comprises other carbohydrates such as xylose, mannose, arabinose, rhamnose, or 4-Omethylglucuronic acid. Mannan: Mannans are the most important constituent of hemicellulose which helps the hemicellulose to bind to the cellulose counterpart. They are widely found as a component in the angiosperm cell wall. β-D-mannopyranosyl units are formed by β-1,4 linkage along with a small ratio of galactans in the linear chains. There are four different types of mannans present: galactomannans, galacto-glucomannans, glucomannans, and linear-mannans. Galactans: Galactans are made of galactose as repeating units linked through α-1, 3, and β-1,6 linkages to form 4-α-D-galactopyranosyl and 3-β-D-galactopyranosyl attached in alternate fashion. They are long polymeric chains not commonly found in all forms of plants. They are majorly found in some algae, seeds, buds, or flowers (Li et al. 2013).

4.3.1.3

Lignin

Unlike cellulose, lignin has an irregular three-dimensional structure with no specific repeating units. Lignin acts as the protective and cementing cover in plants which helps arrange the fibers together to enhance the compactness of the wood thereby making it more resistive. It helps in gluing hemicellulose with cellulose, and it resists the access of enzymes to cellulose by acting as a physical barrier. It is an amorphous organic compound comprising phenylpropanoid units with three different types of phydroxycinnamyl alcohol: coniferyl alcohol, sinapyl alcohol, p-coumaryl alcohol (Fig 4.4c, d). Overall lignification is species specific and is obtained by several crosslinking reactions between the radicals formed by oxidation and resonance delocalization in phenylpropanoid monomeric units. Lignin is synthesized in the plants via shikimic acid pathway. The structural integration of different cellulosic components makes it very recalcitrant to hydrolysis by enzymes. The cellulose and hemicellulose are attached through hydrogen bonds, meanwhile, lignin forms five different types of

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lignin carbohydrate bonds to bind to hemicellulose: γ-esters esters, benzyl ethers, phenyl glycosides, ferulate/coumarate esters, and hemiacetal/acetal linkages (Giummarella et al. 2019; Agrawal and Verma 2020a, b). The biorefineries arena focus on the effective valorization of lignocellulosic materials into valuable products by introducing controlled cleavage of carbon–carbon and carbon–oxygen bonds present in the recalcitrant lignin on the outermost coat. The focus is also maintained on getting rid of various impurities (organic and inorganic).

4.3.2

Starch

Food is stored in plants in the form of starch in seeds, roots, and a little amount in the residual biomass. It is composed of two types of polysaccharides, namely amylose and amylopectin. Amylose is formed by polymerizing D-glucose via α-1,4 linkages linearly, on the other hand in amylopectin, which has branched formation is formed by α-1,4 glycosidic bonds linearly, and α-1,6 glycosidic linkages for branched chains (Whistler and Daniel 1984).

4.4

Enzymes and their Application in Waste

Despite the sustainable availability of biomass for conversion into bioenergy, the process is expensive and time-consuming as it requires lengthy downstream processing for the collection of final products. Before subjecting the different components of waste to hydrolysis, pretreatment of the complex material is required which is the most expensive process in the transformation. It is done to remove the recalcitrant lignin component which remains the major hindrance in exposing the buried cellulose to the saccharification process. The pretreatment involves subjecting the lignocellulosic mass to either high pressure, temperature, or chemical treatment or enzymatic hydrolysis. The chemical pretreatment includes the use of chemicals like organic solvents, concentrated acids/bases, or neoteric solvents which are very harsh and corrosive. Additionally, the process becomes more tedious as many steps are required to separate the final products from the chemicals used in pretreatment. The highly concentrated and corrosive acids and alkali are damaging to the equipment too. There are a lot of unwanted products formed during the process of pretreatment with chemicals that can act as inhibitors to the microbial enzymes used for fermentation. Therefore, the use of hydrolytic enzymes proves to be useful as it poses less cost and also reduces the difficulties faced in the downstream process (Manisha and Yadav 2017). As discussed earlier, the most resourceful feedstock for the production of green fuel is biomass waste, and it has been in regular consideration and experimentation under the biorefinery concept (Azapagic 2014). To switch to industrial symbiosis, i.e., waste from one sector is used as feedstock in another

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Fig. 4.5 A model of biorefinery concept: Biomass feedstocks from various sources are taken as raw material in biorefinery and converted into various forms of bioenergy and other value-added products

industry; therefore, it is very necessary to identify, characterize, and quantify the residues present in the waste stream (Fig. 4.5). Microorganisms and their enzymes have long been core tools in the biofuel refineries, being present in all stages, starting from pretreatment, hydrolysis, and fermentation. They can be introduced as biocatalysts with the substrates where they are produced in situ by microorganisms or they are added ex situ in purified form or as enzyme cocktails. In this section, we will be considering the important enzymes, especially the hydrolytic type in saccharification of polymers into monomers and will also discuss the mechanism of their actions.

4.4.1

Hydrolytic Enzymes Involved in the Valorization of Lignocellulosic Waste

The main source of lignocellulosic biomass is the organic residues obtained from human activities such as agricultural waste and food processing industries. Moreover, solid municipal waste which mostly comprises paper and organics can be included as an important lignocellulosic waste stream for valorization into valuable products. The biopolymers included in lignocellulosic biomass constitute cellulose, hemicellulose, and lignin. The major enzymes required to saccharify these polymers

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are cellulases, hemicellulases, and lignin-degrading enzymes. These enzymes are classified under the single-family of glycoside hydrolases (glycosidases or carbohydrases), E.C 3.2.1, and are involved in the catalysis of O-glycosidic bond hydrolysis (van Wyk et al. 2017; Bhardwaj et al. 2020). The gene corresponding to this class of enzyme is present in all living organisms except in some Archaeans and some unicellular parasites. Glycoside hydrolases cleave the glycosidic bonds via two different mechanisms based on the status of the anomeric configuration during the reaction. The net inversion of an anomeric configuration is achieved as a result of a one-step double displacement reaction between the acidic and basic amino acid groups whereas the retention of the anomeric configuration happens via a two-step double displacement reaction involving acid/base and nucleophilic assistance provided by amino acid residues (Naumoff 2011). All the information concerning genomic, structural, and functional aspects of glycoside hydrolases and their family members is available in a highly curated, knowledge-based database known as the carbohydrate-active enzyme (CAZy) database. This database states that glycoside hydrolases (GHs) are categorized into 135 different families and 14 clans. This classification was based on their overall structural confirmation, amino acid sequence, and function (Lombard et al. 2014).

4.4.1.1

Cellulases

Cellulase is a class of enzymes that hydrolyzes the β-1,4-glycosidic bonds in polysaccharides like cellulose to glucose units and is grouped among glycoside hydrolases (GH). Cellulose from fungus has two domains, namely a catalytic domain, which performs the catalytic activity, and a cellulose-binding domain, which anchors the enzyme to the cellulose substrate. Both the domains are linked together through a linker domain. The hydrolytic action depends upon the synergistic action of three major enzymes, cellobiohydrolase/exoglucanase (E.C 3.2.1.176)/(E.C 3.2.1.91), endocellulase/endoglucanase (E.C 3.2.1.4), and β-glucosidase (E.C 3.2.1.21) (Horn et al. 2012; Kostylev and Wilson 2014). Endocellulases hydrolyze the amorphous area of cellulose to release long-chain oligomers with non-reducing ends which are then acted upon by exocellulase or cellobiohydrolases on the β-1,4glycosidic bonds liberating β-cellobiose. Cellobiohydrolases act on the oligomers from the reducing ends whereas exocellulase act on the non-reducing ends. βglucosidase hydrolyzes the smaller glucans or disaccharide cellobiose into the monomeric glucose (Juturu and Wu 2014) (Fig. 4.6a). Several microorganisms have been found to produce cellulose enzymes. Among bacteria the most important are Clostridium species, Pseudomonas species, and Trichoderma reesei whereas the major cellulose-producing fungi belong to Aspergillus species, Fusarium species, Penicillium species, Schizophyllum commune, and Melanocarpus species. In anaerobic bacteria, cellulose occurs as cellulosomes, an extracellular aggregated enzyme structure. Endocellulase or endoglucanase belonging to the family glycoside hydrolases (GH) 5 comprises a single catalytic subunit made up of 335 amino acids folding into the active enzyme. The structural

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Fig. 4.6 Mechanism of action of cellulases and hemicellulases: (a) Endocellulase, exoglucanase, and β-glucosidase act in synergy for the degradation of cellulose. (b) The mechanism of action of endoxylanase and β-glucosidase on the degradation of xylan to xylose; and (c) synergistic action of β-mannanases, β-mannosidase, and α-galactosidases on the degradation of various units in mannan component of hemicellulose

architecture of endoglucanase has eight (β/α)8 barrel-shaped loops along with a short double-stranded anti-parallel β sheet and three single turns helices. The catalytic substrate-binding site has two glutamate amino acid residues at positions 133 (acidbase) and 240 (nucleophile) which are highly conserved and decisive in the first step of the reaction (Lo Leggio and Larsen 2002) (Fig. 4.7a). Cellobiohydrolases the exoglucanases belong to the family GH 7. The three-dimensional structure of cellobiohydrolases is made up of 431 amino acid residues and exhibits a β-jelly roll structure with two anti-parallel β-sheets to each β-jelly roll. Each β-sheet curves to form concave and convex shapes which are connected through four α-helices. Amino acid glutamine at positions 207 and 212 at the active site of the enzyme participates in the acid-base reaction mechanism (Fig. 4.7b) (Muñoz et al. 2001). β-

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Fig. 4.7 Crystalographic structure of (a) endoglucanase enzyme obtained from Thermoascus aurantiacus (PDB I.D: 1GZJ) (Lo Leggio and Larsen 2002); (b) exoglucanase enzyme obtained from Phanerochaete Chrysosporium (PDB ID: 1GPI) (Muñoz et al. 2001); (c) β-glucosidase enzyme obtained from Bacillus polymyxa (PDB ID: 1BGA) (Sanz-Aparicio et al. 1998); (d) endo-1-4-β-D-xylanases enzyme obtained from Trichoderma reesei (PDB I.D: 1ENX (Törrönen et al. 1994)

glucosidase from the strain Bacillus polymyxa (BglA) exists in a tetramer of dimers arranged in an octameric confirmation. The enzymes form aggregates due to their intracellular localization. The substrate-binding site of the enzyme accommodates Glutamine at 166 and 352 which acts as acid/base and nucleophile in hydrolysis reaction, respectively. The substrate binding is also influenced by Histidine residue at 121 as well as tyrosine residue at 296 positions (Sanz-Aparicio et al. 1998) (Fig. 4.7c). Cellulases have been utilized widely in various industries like brewery distilleries, textile processing, paper pulp industries, detergent production, cattle feed processing, and recently been introduced in the production of biofuels.

4.4.1.2

Hemicellulases

This class of enzyme is also known as hemicellulose degrading enzymes. They are involved in the depolymerization of components present in the hemicellulose portion of lignocellulosic biomass such as galactans, xylans, mannans, and arabans. Mannanases, α-glucuronidases, and α-arabinofuranosidases are widely discussed as well as utilized enzymes. Xylanases: Xylanase enzymes constitute two major enzymes which include endo1-4-β-D-xylanases (EC 3.2.1.8) and β-D-xylosidases (E.C. 3.2.1.37). The endoxylanases hydrolyze xylan into xylooligosaccharides which are further acted upon by xylosidases to yield monomeric xylose (Fig. 4.6b). The structural analysis of endo-β-1,4-xylanase II (XYNII) from Trichoderma reesei reveals that the enzyme exists as a single domain with 190 amino acid residues folded into two anti-parallel β-sheets arranged parallel to each other (Fig. 4.7d). The active site cleft is formed by twisting the β-sheets, and it accommodates two glutamic acid residues at positions 86 and 177 (Törrönen et al. 1994). Some of the accessory enzymes like acetyl xylan esterase (E.C 3.1.1.72), p-coumaric esterase (3.1.1.B10), α-glucuronidases (E.C 3.2.1.139), α-l-arabinofuranosidases (E.C. 3.2.1.55), and ferulic acid esterase (E.C

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3.1.1.73) are necessary to hydrolyze the remaining component or side chains of the hemicellulose structure such as glucuronic acid, galacturonic acid, arabinose, galactose, ferulic and coumaric acids (Bhardwaj et al. 2019; Gírio et al. 2010; Beg et al. 2001). The action of α-glucuronidase, α-l-arabinofuranosidases, and acetyl xylan esterase is to remove acetyl and phenolic side branches whereas p-coumaric esterase and ferulic acid esterase cleave the ester bonds in xylose. The synergistic enzyme activity of all the xylanases proves significant in opening up the xylan component of the lignocellulosic biomass (Moreira and Filho 2016). The presence of xylanases has been found in many organisms ranging from microorganisms like fungi, bacteria, and yeast to crustaceans, insects, and seeds (Beg et al. 2001). Microbial xylanases are preferred over animal sources. The most notified, experimented, and applied xylanases are from the organisms from the genus Bacillus, Chaetomium, Nonomuraea, Arthrobacter, Clostridium, Thermomonospora, Dictyoglomus, Fusarium, Streptomyces, Aspergillus, etc. (Bhardwaj et al. 2019; Sunna and Antranikian 1997). Mannanases: These enzymes are involved in the depolymerization of mannans which are an integral part of the hemicellulose portion of the cell wall. 1,4-β-Dmannohydrolases or β-mannanases (E.C 3.2.1.78) are endo-acting mannanases that cleave the internal glycosidic bonds on the linear chains liberating short oligosaccharides like β-1,4-manno-oligosaccharides. On the other hand, β-1,4-D mannopyranoside hydrolases or β-mannosidases (E.C 3.2.1.25) are exo hydrolases that choose to act on the non-reducing ends of mannobiose to degrade it into individual mannose units. Lastly, β-1,4-D-glucoside glucohydrolases or β-glucosidases (E.C 3.2.1.21) act on the products liberated from the cleavage of glucomannan and galactoglucomannan specifically cleaving the β-1,4-glucopyranose units from the non-reducing terminal (Dhawan and Kaur 2007; Moreira and Filho 2008). αgalactosidases (E.C 3.2.1.22) and acetyl mannan esterases (E.C 3.1.1.6) are some of the accessory proteins which are required to excise the additional side chains or groups present occasionally on the mannans (Malgas et al. 2015) (Fig. 4.6c). Mannanase is classified under different GH families (like GH 1–3, GH 5, 26, 27, 113, etc.). Their primary structure is different while they share common spatial arrangements. They all have a canonical (β/α)8-barrel protein fold in their active site and based on that they have been included in clan GH-A. The central active site cleft contains two glutamate residues at the C-terminal side (Dawood and Ma 2020). Mannanase is the second most important industrial enzyme after xylanases and has been explored in various industries like textile and paper industries, pharmaceuticals, food, feedstock industries, etc. The bacterial degraders for mannanases among Gram-positive bacteria are from Bacillus species, and Clostridia species, whereas from Gram-negative bacteria are from Vibrio, Pseudomonas, Klebsiella, and Bacteroides. Among fungal counterparts Aspergillus, Agaricus, Trichoderma, and Sclerotium, Penicillium species are mostly reported. Actinomycetes from Streptomyces species have also been shown to be mannan degraders (Dhawan and Kaur 2007; Chauhan et al. 2012).

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Fig. 4.8 Lignin hydrolysis by lignin-degrading enzymes: Diagram depicting the mechanism of action of different lignin-degrading enzymes for hydrolyzing lignin component

4.4.1.3

Lignin-Degrading Enzymes

They catalyze the conversion of lignin present in lignocellulosic biomass into small aromatic molecules. Lignin is a phenolic polymer containing phenylpropanoid arylC3 units links. These can be degraded by the synergistic action of two groups of enzymes, namely lignin-modifying and lignin-degrading auxiliary enzymes. The former includes laccase or phenol oxidases (E.C 1.10.3.2), lignin-modifying peroxidases such as lignin peroxidase (LiP) (E.C 1.11.1.14), versatile peroxidases (E.C 1.11.1.16), dye-degrading peroxidases (E.C 1.11.1.19), and manganese peroxidase (MnP) (E.C 1.11.1.13). All the peroxidases have a unique prosthetic group in the form of protoporphyrin IX (Pollegioni et al. 2015). Auxiliary enzymes in lignin degradation on the other hand include glucose oxidase (E.C 1.1.3.4), cellobiose dehydrogenase (E.C 1.1.99.18), glucose oxidase (E.C 1.1.3.4), aryl alcohol oxidase (E.C 1.1.3.7), pyranose 2-oxidase (E.C 1.1.3.10), and glyoxal oxidase (E.C 1.2.3.5) along with some other enzymes like alkyl aryl etherase, and aryl alcohol dehydrogenase (Bilal et al. 2019; Zhang et al. 2020a, b; Levasseur et al. 2008; Agrawal and Verma 2020a, b) (Fig. 4.8). Basidiomycetes white-rot fungus is extensively investigated for the production of these auxiliary enzymes (Garcia-Ruiz et al. 2014). Hofrichter and Ullrich stated an action of a new enzyme heme-thiolate haloperoxidases, catalytically identical to other heme-containing oxidoreductases (cytochrome P450 monooxygenases and

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catalases), as lignin degrading in cultures of Ceriporiopsis subvermispora (Hofrichter and Ullrich 2006). Although the crystal structure and catalytic cycle of all the ligninolytic enzymes are well-reviewed (Chen et al. 2012; Pollegioni et al. 2015; Wong 2009; Janusz et al. 2017), we have attempted to briefly touch on some of the important aspects in understanding the mechanism of these enzymes. Laccases: Laccases fall in the group of oxidoreductases that uses oxygen as an oxidizing agent. The four copper ions at the active center of laccases help the enzyme in oxidizing most of the phenolic and aromatic compounds present in the lignin (Mai et al. 2000; Pollegioni et al. 2015). Some metal ions and organometallic compounds are also been reported as the substrate of laccases (Garcia-Ruiz et al. 2014; Zimmerman et al. 2008). All these four copper ions at the T2/T3 site (the T1 Cu and the tri-nuclear Cu cluster (T2 Cu, T3α Cu, and T3β Cu)) have a different electroparamagnetic resonance which is key to their unique reaction with the random polymeric nature of lignin. The fungal laccases are known to comprise ~520–550 amino acids residues with glycosylation as primary modifications. The three-dimensional structure of fungal laccases demonstrates three tightly arranged cupredoxin-like domains having β-barrel symmetry. The third domain holds the T1 Cu near the surface of the protein, and the T2 Cu, T3 (α and β) Cu are located at the junction of the first and the third domain (Mehra et al. 2018; Sitarz et al. 2016). Laccases are extracellularly, intracellularly as well as periplasmically produced depending on the type of microorganisms producing them. It is found mostly in fungal and bacterial cells. Lignin Peroxides (LiP): LiPs are generally known to oxidize phenolic and non-phenolic organic compounds instead the enzyme specificity is relatively poor. The structural analysis of LiP isolated from Phanerochaete chrysosporium revealed its globular nature. The active site pocket is formed of two domains organized of eight α-helices (major and minor) with restricted β components enclosing a hemechelating ferric ion (Choinowski et al. 1999). The three-dimensional structure of LiP is further stabilized by four disulfide linkages, two calcium ions, and two glycosylation-specific post-translational sites. Although their enzymatic mechanism is similar to other peroxidases in the same class, they stand effective catalytically due to their very high redox potential when it comes to oxidizing the recalcitrant lignin component (Sigoillot et al. 2012). Manganese Peroxidases (MnP): MnP was isolated and studied initially from the fungi Phanerochaete chrysosporium. Supplementation of Mn ions and other organic compounds like 2-hydroxybutyrate, malonate, glycolate, or glucuronate in the growth medium stimulated the production of MnP in white-rot fungus (Mester and Field 2006). These molecules in particular stabilized the structure of the enzyme. A molecule of heme (iron protoporphyrin IX) is sandwiched between the two domains formed by α-helices very similar to that of the structure of LiP. Very close to the heme porphyrin lies the binding site of Mn2+ ion which constitutes one aspartate and two glutamate γ-carboxylic groups. Slightly different from the structure of LiP, MnP consists of five disulfide bridges and two Ca2+ ions. The active cycle of MnP varies

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from that of LiP, in the oxidation of lignin compounds involving the conversion of Mn2+to Mn3+ions (Niladevi 2009). Versatile Peroxidases (VPs): This enzyme represents a cocktail of LiPs and MnPs. It also constitutes a heme porphyrin group close to the catalytic site. The active center is made of 11–12 α-helixes with four disulfide bridges, two Mn (II) binding sites and two Ca(II) binding sites. This enzyme is capable of oxidizing methoxybenzenes and various non-phenolic lignin compounds with high affinity. The multi-step reaction mechanism of VPs is similar to other peroxidases. Dye-decolorizing peroxidase (DyP): These enzymes are different from the classical lipid-modifying peroxidases like LiP, MnP, and VPs. They can decolorize a range of molecules including dyes, β-carotene, and aromatic sulfides (Zámocký et al. 2015). They were first investigated and isolated from the cultures of the fungus B. adusta (Fernandez-Fueyo et al. 2015). DyPs can oxidize non-specifically all peroxidase substrates and also possess hydrolase and oxidase activity. The structural analysis of DyPs demonstrates the presence of two domains. The catalytic site lies in the cavity of the two domains accommodating the heme cofactor (Colpa et al. 2014). Glyoxal Oxidases (GLOX): Lignin components like methylglyoxal and glyoxal can be oxidized by the GLOX. These enzymes proceed with the oxidation of their substrate with the formation of extracellular hydrogen peroxide (Yamada et al. 2014). They are moreover required to regulate the peroxidase activity of the lignin-modifying peroxidases. The active center of GLOX is occupied by a copper ion which helps in the aldehyde oxidation of its substrate (Yin et al. 2015). Aryl Alcohol Oxidase (AAO): This enzyme is a member glucose-methanol-choline oxidase/dehydrogenase family. Structurally, this enzyme is a monomer with two domains non-covalently bound with the FAD cofactor (Fernandez et al. 2009). AAO substrates include various aryl-alcohols (phenolic and non-phenolic), aliphatic (polyunsaturated) primary alcohols, or aromatic secondary alcohols present in the lignocellulose biomass. It also oxidizes the radical intermediates produced by laccase enzymes like guaiacol, sinapic acid, etc. (Mathieu et al. 2016). The oxidative dehydrogenation reaction of AAO is an NADP-dependent reaction and produces H2O2 (Ferreira et al. 2010). Pyranose 2-Oxidase (P2O): These oxidoreductases are involved in the oxidation of aldopyranose compounds. It is produced periplasmically and transported in membrane-associated vesicles (De Koker et al. 2004; Prongjit et al. 2009). They are homotetrameric and have three major conserved regions, namely the binding site for FAD, the substrate-binding region, and the flavin attachment loop. The threonine hydroxyl of Thr169 present at the active site is very important for the oxidation of sugars and flavin molecules. The P2O catalyzes the oxidation of its substrate at the C2 position via a Ping-Pong type reaction mechanism at pH 7 (two half-reactions). The end products are the 2-keto-sugars and hydrogen peroxide. First, a hydride equivalent from the sugar substrate is donated to the protein-bound flavin with the generation of a reduced FAD (FADH) and the 2-keto-sugar, and secondly, a reduced flavin is oxidized by donating two electrons to O2 thereby forming H2O2 (Pitsawong et al. 2010).

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Hydrolytic Enzymes Involved in the Valorization of Food Waste

Food waste origin is marked starting from the agriculture sector, packaging, and retail, finally till household consumption. These waste streams generated are rich in biodegradable organic matter, which due to their peculiar chemical characteristics like high biological and chemical oxygen demand, content-rich in carbon and nitrogen, are deleterious to the environment when discarded in landfills and aquatic streams. They may cause aquatic pollution, harmful toxic leaching into ground and surface water, altered soil composition, etc. The food processing industries and so their waste is categorized based on the food material they process. Baiano et al. (2014) assessed and estimated the approximate percentage of waste generated by different food processing units or industries (Baiano 2014). According to them, the beverages manufacturing industry generates 26% of food waste, dairy industries make up 21%, fruit/vegetable, cereal edible oils manufacturing and processing as well as meat and fish product processing and preservation industries constitute (12.9%), (3.9%), (8.4%) of food waste, respectively. However, valorization of these compounds to revenue streams like biofuels, other food, and non-food commodities is possible by introducing enzymes catalysis at various steps of their conversions (Andler and Goddard 2018). For instance, esterification of different waste components like oil, sugar, starch, and even flavonoids help in enhancing their values through converting or valorizing them into value-added products like biodiesel, surfactants, biodegradable plastics, and this indirectly prevents their direct disposal to wastewater treatment facilities. Pectinases: The industries processing vegetables and fruits are rich in crude dietary fibers, carbohydrates, polyphenols, flavonoids, triglycerides, or plant-based fatty acids, etc. The by-product of these industries can be widely valorized through enzymatic and also through other physico-chemical extraction into value-added revenue streams like novel pharmaceuticals, animal feeds, etc. (Mourtzinos and Goula 2019; Fierascu et al. 2020). For instance, soluble and insoluble dietary fibers from citrus fruit pulp have been investigated by a group of researchers to replace fat content in ice cream because of their high phenolic and carotenoid content and more importantly because of their high water and fat retention capacity (de Crizel et al. 2014). Fierascu et al. (2020) have also extensively reviewed the current opinions on utilizing the waste generated from fruits processing industries into useful commodities (Fierascu et al. 2020). Pectinases are important fibrinolytic enzymes, and they are widely utilized in beverages industries for clarification as well as enhancement of color purposes. These enzymes help dissolve pectin structures (Micheli 2001) and are categorized based on their mechanism of bond cleavage: (1) pectin esterases and (2) depolymerase enzymes. Depending upon the substrate on which the pectinases act, pectin esterases are of two types, i.e., pectin acetylesterases (E.C 3.1.1.6) and pectin methylesterase (E.C 3.1.1.11). Pectin depolymerase, on the other hand, are hydrolases, for example, polygalacturonase, PG (E.C 3.2.1.15), and lyases or transeliminase comprising pectin lyase, PNL (E.C 4.2.2.10), and pectate lyase, PL

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Fig. 4.9 Action of pectinases: Mechanisms of action of polymethylgalacturonase, polygalacturanase, pectin esterases, polygalacturanase, and pectin lyase on their specific pectin substrate

(E.C 4.2.2.2) (Sharma et al. 2013). Pectin methylesterases (E.C 3.1.1.11) convert the methyl groups into pectic acid and the depolymerase enzyme further disintegrates pectic acid into simpler carbohydrates (Fig. 4.9). Pectinases in common consist of only one domain termed parallel β-helix making a right-handed cylinder. The structure contains three parallel β-sheets. These βsheets arrange themselves to form a prism-like structure with two β-sheets forming an anti-parallel sandwich while the third one remains perpendicular to the axis. The active site of the enzyme is formed on the exterior side of the parallel β-helix. This structural model is most similar and closest to the structures demonstrated in the referred articles through crystallographic studies in pectinases from various strains (Jenkins et al. 2001; Pickersgill et al. 1994, 1998; Petersen et al. 1997). Inulinase: Inulinase acts upon the β-2,1-linkage between the fructose units present in the inulin molecule. The polyfructose chain is terminated with a glucose unit attached with α-1, β-2-glycosidic linkage. Inulin is a stored carbohydrate found commonly in roots and tubers of plants like onion, garlic, and also there are reports of bacterial inulin which are comparatively highly branched in nature. Inulinase enzyme is classified into two classes: an endo-inulinase called 2,1-β-Dfructanfructohydrolase (E.C 3.2.1.7) and an exo-inulinase called β-Dfructanfructohydrolase (E.C 3.2.1.80) (Neeraj et al. 2018). The structural analysis of inulinase from Aspergillus awamori reveals the presence of two catalytically important residues (Asp41 and Glu241) at the substrate-binding site of the enzymes. These residues play a vital role in the double displacement reaction at the initial hydrolysis step (Nagema et al. 2004). Modern nutrition prospects recommend artificial sweeteners in the place of sucrose in diet to people looking for maintenance of weight/weight loss and lowering blood sugar levels in diabetes. Fructose products

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have also made their place as sweeteners in prebiotics popularly known as Greek yogurt. Here’s where Inulinase yields its importance in industries generating fructose or fructose oligosaccharides as artificial sweeteners. Inulinase has also been established in the production of a plethora of commercially important products like citric acid, tetrahydrofuran, mannitol, sorbitol and 2,3-butanediol, etc. (Chi et al. 2009). Lipolytic Enzymes: Wastewater generated from the slaughterhouse, poultry, and fish farms are rich in biodegradable organic matter like oil and grease waste which can result in high BOD and COD. As a consequence, there is an increase in the growth of filamentous microorganisms (bulking), floating and clogging of sludge in the treatment plant, and unpleasant odor. This demands the hydrolysis of the organic matter before its release into the treatment plants for further processing. Even though there are various methods like dissolved air flotation systems, tilted plate separators, grease-trap, and physical-chemical treatment (aerobic and anaerobic pretreatments) employed for the removal of these biodegradable organic matters from the wastewater, their proper implementation counters several setbacks. Therefore, effluents from several origins can be subjected to enzymatic hydrolysis by lipase. Treatment of domestic wastewater rich in oil and grease with lipase obtained from Candida rugosa (Jaeger and Reetz 1998) and Pseudomonas aeruginosa (Dharmsthiti and Kuhasuntisuk 1998) has been vastly investigated. Lipases are chemically known as triacylglycerol hydrolases (E.C. 3.1.1.3) which catalyze the hydrolysis of triacylglycerol. The active site of lipases is decorated with amino acids like serine, aspartate or glutamate, and histidine (Mateos et al. 2021). They exist as monomeric proteins folded to form β-sheets in the center enclosed by α-helix. The lipase activity is considered maximum at the oil-water interface which is dependent upon its change in conformation from closed to open form upon coming in contact with a hydrophobic surface. Apart from hydrolysis, lipases can catalyze other reactions like esterification, transesterification, acidolysis, and aminolysis. Lactases: Dairy industries are also major contributors of proteins, fatty acids, and lactose in liquid waste. The major portion of which is considered to be deproteinized cheese whey obtained from cheese producing industries all over the world, which despite no toxic content is a major concern to environmental preservation and safety (Lappa et al. 2019). The lactose component in cheese whey has been elaborately investigated for the production of several commodities like bioethanol, artificial sweeteners, green plastics/polyhydroxyalkanoate polyesters, etc. (Koller et al. 2012), and there are reports of its valorization into disinfectants, electron donors for electricity generation through single chamber microbial fuels cells as well (Banaszewska et al. 2014; Pescuma et al. 2015; Yadav et al. 2015). Lactases are known as β-D-galactohydrolase/β-galactosidases (E.C 3.2.1.23), and they catalyze the cleavage of lactose into glucose and galactose. The most demanding prebiotics, lactulose, and galacto-oligosaccharides (GOS) are produced by lactose through the action of β-galactosidases. Lactose is transgalactosylated by β-galactosidases to produce a mixture of non-digestible forms of mono-, and oligosaccharides that form the GOS. In the reaction, lactose acts both as the donor and acceptor of

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transglycosylated galactose. Whereas in lactulose synthesis, lactose only acts as a donor of galactosyl glucose, and the fructose acts as an acceptor. Protease: Proteases are classified into different groups based on the activity of proteases under various parameters like acidic, alkaline, or neutral conditions as well as the composition of their substrate-binding site (Panda et al. 2013). Alkaline serine protease (E.C.3.4.21–24.99) is most active at pH ranging from neutral to alkaline (Singhal et al. 2012; Varela et al. 1997). Other proteases are the neutral and acidic proteases (Razzaq et al. 2019). Proteases are proven to be a significant tool in bioremediation as well as valorization of waste to value-based products (Page and Cera 2008). The most exploited and studied microorganism for the production of alkaline protease and neutral proteases are Bacillus species and acidic proteases are known to be produced by the fungus. These proteases can be efficiently inactivated by PMSF (phenyl methane sulfonyl fluoride). Protein-rich waste is responsible for increasing the biological oxygen demand of aquatic waste. As discussed in earlier sections, the waste aquatic stream is drowned with effluents from food processing industries which constitutes waste from dairy products, waste from poultry, and fish industries as well as textile and leather industries. Leather industries involve alkaline protease with keratinolytic activity to hydrolyze keratin content present in the hair residues for increasing the surface area of the skin. They are also used in bating and clearing undesirable pigments during the preparation of clean skin and hides (Bhaskar et al. 2007; Shankar et al. 2011). Alkaline protease preparations of Bacillus species are enormously used in poultry industries to get rid of the feather waste generated in the slaughterhouse. The keratinolytic capability of these proteases has been best utilized in cleaning drainage pipes clogged with hair residues. A cocktail of protease preparation from Streptomyces species, Bacillus subtilis, and Bacillus amyloliquefaciens along with thioglycolate mixture is available commercially in the market. Alkaline proteases find a crucial place in the degradation of plastics as well as X-ray photographic sheets, particularly for the silver recovery. The active site of the alkaline protease comprises catalytic triad formed by Aspartate and Histidine residues along with Serine residue. Proteases based on their structural and sequence similarity can be obtained and assessed from the database called MEROPS (Rawlings et al. 2006) database. Table 4.1 comprehensively covers the application of different enzymes in the valorization of waste.

4.4.3

Hydrolytic Enzymes in Biodegradation and Valorization of Non-biodegradable Plastics Waste

The monomers utilized for the preparation of most plastics, ethylene, and propylene are derived from petroleum. The most commonly used are polyethylene, polystyrene, polyurethane, polypropylene, polyvinyl chloride, poly (ethylene terephthalate), etc. Out of the total production worldwide, only a small fraction around 20% is getting recycled, thus plastic remains a long-lasting, and major threat to the

Name of the enzymes Cellulase

Xylanase

Sl. No. 1

2

Industry Pulp and paper industry, Textile industry Agricultural, and Food Industry Other

Agricultural, Food Industry, Pulp and paper industry, Textile industry, and Pharmaceutical industry

Microbial source Bacteria: Clostridium species, Pseudomonas species, Bacillus species Fungus: Aspergillus species, Penicillium species, Melanocarpus species, Trichoderma species, Schizophyllum commune Actinomycetes: Cellulomonas species, Streptomyces species, Thermomonospora species

Bacteria: Arthrobacter species, Geobacillus species, Pediococcus acidilactici, Bacillus species, Paenibacillus species, Rhodothermus marinus, Staphylococcus aureus, Pseudomonas species Fungus: Talaromyces species Thermomyces lanuginosus, Thermoascus sp,

Application Deinking, pulping, bleaching, improve the quality and strength of pulp fibers, reduce the use of energy and chlorine, production of biodegradable sanitary napkins, papers, etc. Bio-stoning of jeans, softening of garments, biopolishing of textile fibers, enhancing quality, stability, and absorbance property of textile fibers, removal of excess dye from fabrics, and restoration of color brightness. Improve seed germination, plant growth and flowering, disease and plant pathogen control, and improve soil quality. Bioethanol production, bio-detergent production Improve fermenting process, improve animal feed digestibility, biogas, and bioethanol production, clarification, and maceration of fruit juice, brewing, hydrolysis of agroresidues. Bio-bleaching of pulp, Deinking Textile processing, desizing, and bioscouring. XOS production.

Table 4.1 Listed applications of different hydrolytic enzymes in the valorization of waste and their microbial sources

Beg et al. (2001), Bhardwaj et al. (2019)

Reference Pere et al. (2001), Kuhad et al. (2011), Singh et al. (2016)

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Mannanase

Aryl alcohol oxidase

Lactase

Alkaline protease

3

4

5

6

Bacteria: Sphingobacterium species Fungus: Thermothelomyces thermophilus, Coprinopsis cinerea, Phanerochaete chrysosporium Bacteria: Bifidobacterium infantis Fungus: Lactobacillus thermophiles, Kluyveromyces fragilis, Aspergillus foetidus Bacteria: Bacillus pumilus Fungus: Streptomyces fungicidicus, Myceliophthora species

Melanocarpus albomyces Chaetomium species, Fusarium species Humicola species Paecilomyces sp., Scytalidium sp. Bacteria: Agaricus bisporus, Aspergillus species, Bacillus species, Bacteroides species, Clostridium species Fungus: Aspergillus species, Agaricus species, Trichoderma species, Sclerotium species

Leather Industry, Cosmetic sector, Pharmaceutical Industry and

Food and beverages, Polymer industry, and Pharmaceutical industry Food and beverages

Pulp and paper industry, Textile industry, Agricultural and Food Industry

Helps in effective bating. Formulation of cosmetic products. Development of ointment compositions, gauze, and new bandage material. Detergents and silver recovery.

Bleaching of softwood pulps using enzymes, eliminates the use of chlorine and hydrogen peroxide, and enhance the brightness of paper. Desizing and bioscouring, reduce the viscosity of the print paste. Oil extraction from coconut, food additives, oil drilling, improve the nutritional value of animal feed. Food flavors, e.g., vanillin. Production of bio-based polymer. Production of health beneficial compounds, e.g., to cuminaldehyde (therapeutic effects against cancer and diabetes). Production of lactose-free products reduces the crystallization of ice creams and condensed milk

The Pioneering Role of Enzymes in the Valorization of Waste: An. . . (continued)

Baweja et al. (2016), Sharma et al. (2019), Razzaq et al. (2019)

Saqib et al. (2017), Porta et al. (2010)

Urlacher and Koschorreck (2021), Serrano et al. (2019), Li and Jiang (2004)

Dhawan and Kaur (2007)

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8.

Esterase

7

Microbial source

Bacteria: Lactobacillus casei, Pseudomonas species Fungus: Aspergillus niger Lignin-degrading enzymes Fungus: Lignin perP. chrysosporium Trametes oxidase, versicolor, Phlebia radiate, Manganese Trichoderma viride, Trametes peroxidase suaveolens, Phanerochaete sordid (MnP), and Bacteria: Glycol oxiBacillus anthracis and Bacillus cereus dase Fungus: (GLOX) Rigidoporus Lignosus and Ceriporiopsis subvermispora Bacteria: Streptomyces lividans Fungus: P. chrysosporium, Pycnoporus cinnabarinus, Bjerkandera adusta, Dichomitus squalene

Name of the enzymes

Sl. No.

Table 4.1 (continued)

LiP and MnP: Decolorize dyes and degradation of a xenobiotic compound. LiP: Decolorize synthetic melanin an alternative to hydroquinone cream. Lip and MnP: Biopulping, biobleaching, delignification in the paper industry. Lip and MnP: Polymerization of acrylamide into a thermoplastic resin (polyacrylamide). LiP and MnP: Delignification of lignocellulose. GLOX: Oxidize aldehydes to a variety of organic acids, carboxylic acids, pyruvic acid, oxalic acid, and formic acid, source of H2O2 production during lignin degradation

Production of chiral drugs, detoxifying toxic residue. Reducing pitch problems during paper manufacture.

Other applications Pharmaceutical industry, Paper and pulp industry

Textile industry, Cosmetic industry, Pulp and paper industry, Polymer industry, Food industry, and Other application

Application

Industry

Marco-Urrea and Reddy (2012), Abadulla et al. (2000), Joon Sung (2020), Agarwal et al. (2018), Iwahara et al. (2000), Daou and Faulds (2017), Falade et al. (2017), Januz et al. (2017)

Panda and Gowrishankar (2005)

Reference

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Pectinase

Insulinase

9

10

Bacteria: Streptomyces species Fungus: Kluyveromyces marxianus, Aspergillus niger.

Bacteria: Bacillus subtilis, Saccharomyces cerevisiae Fungus: Aspergillus niger, Rhodotorula species Food and beverages

Food and beverages, Textile industry, and Other applications

Reducing the viscosity and turbidity of freshly collected juice, removing the mucilage from the coffee beans. Bioscouring of cotton fibers, degumming, and retting of fiber crop, oil extraction. Purification of plant viruses. Production of metabolites, e.g., lactic acid, sorbitol syrup, and high fructose syrup and oligosaccharides Singha et al. (2017)

Nighojkar et al. (2019), Sharma et al. (2019)

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environment, especially to the aquatic streams (Kaushal et al. 2021). Plastics are categorized into two ways: (1) Thermoplastics whose chemical composition remains unchanged at high temperature and have mostly linear carbon chain backbone and (2) Thermosets which are made of other elements along with carbon possess highly cross-linked anatomy and their chemical conversion at high temperatures is irreversible. One of the most common thermoplastics used worldwide is polyethylene (PE). It is composed of ethylene as a monomer unit and is highly crystalline in structure which makes it recalcitrant to biodegradation. Based on the pattern of the linear chain and different densities, PE can be low-density PE (LDPE), high-density PE (HDPE), and also low molecular weight PE (LMWPE) Polypropylene (PP), yet another thermoplastic made by polymerizing propylene gas. PP differs from PE in having a methyl group instead of one hydrogen atom at an alternate carbon atom in the linear carbon backbone. This chemical structure gives it more rigidity in comparison to PE. Both these plastics are included in the category of polyolefin due to their inert and resistant nature to most heating, biological and chemical treatments. It is abundantly utilized as packaging plastics in various industrial sectors (Zheng et al. 2005). Polyvinyl chloride is yet another synthetic plastic that is used in rigid or plasticized forms. They are formed by polymerizing vinyl chloride or chloroethene in linear form. The pollution caused by PVC plastics is noticeable as burning these plastics emits hydrogen chloride fumes which pose serious health hazards. They are more prone to microbial degradation in comparison to other plastics due to the high percentage of plasticizers added to them (Webb et al. 2000). Polystyrene is synthesized by polymerizing styrene as repeating units. They could be thermoplastics or thermosets. They are also widely used in packaging industries due to their foam-like appearance (Tokiwa et al. 2009). Polyurethane and PET (polyethylene terephthalate) both have improved thermostability as they are hetero-atomically branched. PET is the most abundantly produced and used plastic in the modern era. Researchers have claimed that global warming caused by enormous CO2 emissions and promiscuous usage of PET plastics are two of the most alarming situation in the biosphere (Wang et al. 2020). It is a high molecular weight thermoplastic composed of terephthalate (TPA) and ethylene glycol (EG) via an ester bond. This polymer has great tensile strength, and durability and its production cost are also low. Their structure contains large aromatic rings which make them rigid and resistant to biodegradation (Webb et al. 2013). Polyurethane has heteroatoms with carbamate linkage which could be either ester or ether bonds. They form the major constituents of microplastics making them the most concerning issues in the aquatic system (Shah et al. 2013). Mitigation of these plastics from the environment is carried over by certain physical and chemical methods like incineration, recycling, and dumping them into landfills. All of these methods are not environmentally friendly and they are not even costeffective. The introduction of plastic hydrolyzing enzymes has opened up hopes for eco-friendly treatments to get rid of this dire environmental pollutant (Verma et al. 2016) (Table 4.2). Varieties of microorganisms like fungi, bacteria, actinomycetes, and algae have been investigated as well as reported to exhibit plastic polymer degrading capacity. Plastic degrading enzymes are mostly obtained from microbial organisms, and therefore they are studied under two categories: intracellular and

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Table 4.2 Plastic degrading enzymes and their microbial sources Sl. No 1

Enzyme Laccase

Microorganism Rhodococcus ruber C208 (Mesophilic bacteria) Bacillus cereus (Bacteria)

2

Manganese peroxidase

3

Lignin peroxidase

4

Alkane hydroxylase

5

PETase

6 7

Cutinase-like enzyme/ IsPETase Lipase

8

Polyurethanase

9

Protease and esterase

Phanerochaete chrysosporium ME446 (White-rot fungus) IZU-154 (Fungi) Bacillus cereus (Bacteria) Penicillium simplicissimum (Fungi) Streptomyces (Bacteria) Phanerochaete chrysosporium MTCC787 (Fungi) Recombinant AH from Pseudomonas species E4 expressed in Escherichia coli BL21 (Bacteria) Chlamydomonas reinhardtii (Green algae) Ideonella sakaiensis 201-F6 (Bacteria) Pseudomonas chlororaphis (Bacteria) Serratia marcescens (Bacteria) Pseudomonas fluorescens (Bacteria) Pseudomonas chlororaphis (Bacteria)

10

Polyurethanase (PUase) Polyhydroxyalkanoate depolymerase

11

12 13

Polyester Polyurethane (PUR) esterase Phenylacetaldehyde Dehydrogenase

Curvularia senegalensis (Fungi) Alcaligenes faecalis (Gram-negative, rod-shaped bacteria) Comamonas acidovorans TB-35 (Bacteria) Pseudomonas fluorescens ST (Bacteria) Pseudomonas putida S12 (Bacteria) Xanthobacter species 124X (Mesophilic bacteria)

Plastic substrate Polyethylene (PE)

References Santo et al. (2013), Vimala and Mathew (2016) Iiyoshi et al. (1998), Sowmya et al. (2015)

Jeon and Kim (2015), Mukherjee and Kundu (2014) Yoon et al. (2012)

Polyethylene terephthalate (PET)

Kim et al. (2020)

Polyurethanes

Stern and Howard (2000) Mankoci et al. (2019) Hung et al. (2016), Shah et al. (2008), NakajimaKambe et al. (1995) Crabbe et al. (1994) Gamerith et al. (2016)

Polyester polyurethane Polystyrene

Akutsu et al. (1998) Oelschlägel et al. (2018)

Han et al. (2017)

(continued)

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Table 4.2 (continued) Sl. No 14

Enzyme Cytochrome P450 CPY152A1

15

Cytochrome P450 CPY152B1

16

AlkB (alphaketoglutarate-dependent hydroxylase Alkane monooxygenase

17

18

Hydroquinone peroxidase

Microorganism Bacillus subtilis (Grampositive, catalase-positive bacteria) Sphingomonas paucimobilis (Gram-negative bacteria) Pseudomonas putida GPo1 (Bacteria) Geobacillus thermodenitrificans NG80–2 (Thermophilic bacteria) Azotobacter beijerinckii HM121 (Lignin decolorizing bacteria)

Plastic substrate

References Shoji et al. (2007)

Fujishiro et al. (2012) Hou and Majumder (2021) Li et al. (2008)

Nakamiya et al. (1997)

extracellular enzymes. Extracellular enzymes are involved in the depolymerization of long-chain polymers into smaller fractions, viz. oligomers, dimers, etc. Whereas the intracellular enzymes participate in the final conversion of intermediates into the forms which can be assimilated by the microbes as a sole source of carbon. As a result of this process, a valuable emission gas, i.e., methane is released as metabolic products which can be used as fuels and can further be utilized as precursors for the production of organic acids (Amobonye et al. 2021). Furthermore, the wax moth Galleria mellonella is known to depolymerize plastics with the help of their gut microbiota containing the fungus Aspergillus flavus (Zhang et al. 2020a, b). All these enzymes involved in degrading plastics are hydrolases that catalyze the cleavage reaction in the presence of water (Müller et al. 2005). Esterases, cutinases, laccases, lipases, and PETases are the most extensively studied hydrolytic enzymes concerning the degradation of plastics. Microbial valorization of plastics into value-added chemicals is elaborately reviewed in Ru et al. (2020). The reviewer explained elaborately the microbial metabolic pathway involved in the depolymerization of ester/urethane-containing plastics, aromatic plastics, and linear aliphatic plastics into its monomer constituents and their further assimilation by microbes for the production of value-added chemicals. The chemical structure such as linkages in petro plastics, linearity or branching in carbon chain, type of linkage (ester, ether, or carbamate linkage between the monomers), presence of hydrophobic functional groups, and physical properties like rigidity (crystalline/amorphous) and density plays a significant role in engineering enzymes suitable for biodegradation (Mohanan et al. 2020). Ongoing and present studies for identification of plastic degrading microorganisms and modification of these microbial enzymes through genetic engineering provides a wide opportunity to efficiently recycle or remove

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plastic from the environment. Yet advantageous is when these plastics can be converted into more valuable and marketable products.

4.5

Recent Biotechnological Trends in Increasing the Efficacy of Enzymes in the Waste Valorization

Enzymes have been central in various industries like food processing, beverages distilleries, leather, textile, and paper industries for a long time (Sheldon and Woodley 2017). Their involvement in the production of biofuel and value-added products is tremendously surging in recent times (Chapman et al. 2018). Consequently, for obtaining optimal bioprocesses involving enzymes, further enhancement of enzyme stability and functionality is indispensable. Biotechnological breakthrough offers a great deal for enhancing the power of existing enzymes as well as identifying newer enzyme candidates.

4.5.1

Techniques to Decipher Newer Biocatalyst Candidates

Conventionally, enzyme discovery was done by cultivating microorganisms, fractionating cell-free extract, followed by the screening of the enzyme activity, and then recovered enzymes through purification are subjected to mass spectrometric analysis after trypsin digestion. The identified short peptides are then utilized to decipher the corresponding gene from the genomic DNA. Although these methods are dependent on the use of cultivable microorganisms, a significantly important set of enzymes were discovered using this process. State-of-the-art tools are now assessable to scrutiny and tap the vast microbial biodiversity present in nature (Rinke et al. 2013). The introduction of omics such as metagenomics and metatranscriptomics presents a big potential to analyze the diversity of complex microorganisms. These tools help in the development of genomic libraries from environmental DNA for function or sequence-based similarity screening of the enzymes (Gilbert and Dupont 2011; Uchiyama and Miyazaki 2009). For example, a collection of hydrolytic enzymes such as amylase, lipase, oxidoreductase, and epoxide hydrolase have been deciphered using this technique (Knietsch et al. 2003; Rondon et al. 2000). Several bioinformatics strategies such as in silico data mining, the Catalophore™ approach, and de novo enzyme design tools are also helpful in this process (Handelsman 2004). Progressive success in developing methods for genome sequencing like next-generation sequencing has opened up newer approaches to hunt for putative enzymes. Here, genome hunting is based on either searching for the open reading frame or homology alignment of sequences.

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Isolation of Enzymes from Extremophiles

Enzymes that can withstand extreme parameters like higher temperature and pH as well as harsh chemicals like a high concentration of salts, metal ions, organic solvent, etc. have great value at industrial levels. There is a large diversity of organisms in the extremophilic regions and as most of these organisms have not been yet cultivated in pure cultures, the characterization of their enzymes from them is comparatively difficult (Pikuta et al. 2007; Cavicchioli et al. 2011). The bacterial isolates obtained from the extremophiles have displayed properties of different hydrolytic enzymes such as amylase, protease, lipase, and xylanase. Extreme thermophiles are widely present in bacterial species like Thermus, Thermotoga, Clostridium, and Bacillus. Pyrococcus, Thermococcus, or Methanopyrus belongs to hyperthermophilic Archaea. Hydrolytic enzymes such as Amylase, Xylanase, Lipase, and Protease enzymes are isolated from some halophilic bacterial species like Halobacterium, Halobacillus, and Halothermothrix (Moreno et al. 2012).

4.5.3

Genetic Engineering or Recombinant DNA Technology for the Production of Recombinant Protein in a Microbial Host

To obtain desired efficacy in the expression of enzymes, gene-based technology like recombinant DNA technology can be used. In this technology, the desired gene or gene of interest is inserted into the organism via an appropriate vector. The gene of interest can be manipulated through the addition of the desired sequence in the endogenous gene or deletion or knockout of undesirable sequence through recombining genes and elements. Rational redesigning and direct evolution are the two different methods that are adapted for modifying enzymes to their desired characteristics. Rational redesigning utilizes site-directed mutagenesis to target amino acid substitution effectively at the active site of the protein for evolving the enzyme into a more efficient one. Whereas the direct evolution method includes repeated oligonucleotide-directed mutagenesis, random mutagenesis through errorprone polymerase chain reaction (PCR), or modification through chemical agents (Manisha and Yadav 2017; Wiltschi et al. 2020) (Table 4.3).

4.5.4

Immobilization of Enzymes

The major confrontation in enzyme technology and its application in industries is bulk production and the question of reusability. These problems can be easily dealt with the immobilization technique. Enzymes can be immobilized by tethering or encapsulating them in an appropriate material that has desired physical, chemical,

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Table 4.3 Genetically engineered microbes for increasing the efficacy of expressed hydrolytic enzymes in the valorization of waste Sl. No. 1

Enzymes Cellulase

2

β-glucosidase

3

Xylanase

4

Laccase

5

Lipase

6

Lipase B

7

Pyranose 2-oxidase Endoglucanase

8

Improved properties Increased the inherent ability of Lactiplantibacillus plantarum’s lignocellulose degradation Improve enzyme activity and thermostability Tm improved by 25  C 3-fold improved kcat and thermostability 2-fold increase in amidase activity 20-fold increase in half-life at 70  C

Increased thermostability and the catalytic properties Increase hydrolytic activity on cellulosic substrate

Organism Thermobifida fusca

References None and Yadav (2017)

Trichoderma reesei

Lee et al. (2012) Yang et al. (2017) Mollania et al. (2011) Fujii et al. (2005) Siddiqui and Cavicchioli (2005) Spadiut et al. (2009) Srikrishnan et al. (2012)

Thermotoga thermarum Bacillus HR03 Pseudomonas aeruginosa Candida antarctica

Trametes multicolor Thermoascus aurantiacus

electrical, or mechanical properties. These materials increase the stability as well as efficacy in terms of better catalytic activity of the immobilized enzymes. Moreover, immobilization can reduce the steps required to separate them from the reaction mixture as well allows substantial reusability without affecting the activity allowing them to be compatible in a continuous process. To examine the enzyme activity of immobilized enzymes two kinetic parameters, namely the Michaelis constant, Km, and maximal reaction velocity, Vmax is often examined and compared with the non-immobilized counterpart. The three widely used immobilization techniques are encapsulation or entrapment, carrier-bound attachment, and the formation of cross-linked enzyme aggregates (CLEAs). Encapsulation or entrapment technique, as the name suggests is the immobilization of enzymes using material of varying degrees of porosity and permeability (Bezerra et al. 2015; Jesionowski et al. 2014). Various materials such as a variety of carriers, for example, sol–gels, hydrogels, polymers as well as nanomaterials have been experimented with immobilization of enzymes via encapsulation technique. Carrier-bound immobilization of enzymes is done by physisorption or chemisorption of enzymes on prefabricated organic or inorganic materials. Materials like metal oxides, nanomaterials, ceramic, or silica gels are used for this purpose. Chemisorption technique is preferred over physisorption as covalent attachment reduces the chances of enzyme leaching from the matrix. CLEAs technique is a very recently utilized technique where the soluble enzyme is aggregated using precipitating agents like alcohol, acetone,

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ammonium sulfate, etc., and then subsequently cross-linked or co-polymerized with cross-linking agents like glutaraldehyde (Sheldon 2010). Hence, immobilizing enzymes can be well exploited for obtaining desired characteristics to bring increased efficacy in the use of enzyme technology in the industrial arena as well as biorefineries.

4.5.5

Cell Surface Engineering

Genetic technological advances have let us modify microorganisms to express our genes of interest (Table 4.3). Cell surface engineering is another biotechnology tool where the particular microorganism is tailored to express the desired number of enzymes on its surface. This is technique is advantageous in biorefineries where there is a need for multiple enzymes for the conversion of biomass to biofuels. The most utilized and engineered organism includes Saccharomyces cerevisiae. Naturally, yeast can ferment sugars to produce alcohols, but it does not possess the enzyme required for saccharification of complex sugars present in the cell wall of the biomass. Cell surface engineering enables the arming of yeast and other organisms with a cocktail of enzymes that help the production of biofuels and other valueadded products (Kuroda and Ueda 2013; Ueda 2016; Ueda and Tanaka 2000).

4.6

Conclusions

Developing sustainable approaches toward building a circular economy and safeguarding mother nature has become the need of the hour. Toward achieving this, biomass waste from various sectors like agro-forest, solid municipal waste, food manufacturing, and processing industries can be utilized and profitably and competently converted to bioenergy and value-added products. To reduce the losses incurred by the use of traditional chemical processes in the treatment of waste biomass, enzymes are introduced for the economic and easier hydrolysis of the different waste components. The major portion of waste generated is composed of biodegradable biomass as well as non-biodegradable plastics. Different microbial sources of hydrolytic enzymes are exploited for the valorization of these wastes into value-added products and bioenergy. Cellulases, hemicellulases, lignin peroxidases, pectinases, amylases, proteases, etc. enzymes are widely utilized in this concern. The biorefinery concept is structured to use the by-product from one industry as feedstock for another industry to produce more value-added goods in addition to conversion of waste to bioenergy. Various technological advances like genetic engineering, cell surface engineering, and immobilization of enzymes are exploited to increase the efficacy of the hydrolytic enzymes obtained from microbial sources. Identification of new sources of these enzymes through metagenomic analysis is relevant and very necessary to keep the reservoir filled. In addition to that further

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improvement of these enzymes could be done through techniques like metabolic engineering and chemical modification of the enzyme. Acknowledgments The authors gratefully acknowledge the financial and infrastructural support from the Indian Institute of Technology Palakkad and Department of Science and Technology (DST)-SERB, Govt. of India for conducting research work. AB is supported by the National Postdoctoral fellowship granted by DST-SERB, (PDF/2020/001950) Govt. of India, RS is supported by a scholarship from Council of Scientific and Industrial Research, MHRD, Govt. of India (09/1282 (0003)/2019-EMR-I) and SC is supported by DST-INSPIRE (IF-18022) fellowship, Govt. of India. Competing Interests There is no conflict of interest.

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Chapter 5

Thermochemical Conversion of Biomass into Value-Added Materials for Effluent Treatment Applications Nethaji Sundarabal, Vairavel Parimelazhagan, Suganya Josephine Gali Anthoni, Praveen Kumar Ghodke, and Sivasamy Arumugam

Abstract Lignocellulosic biomass has been explored for the synthesis of various value-added materials due to its wide availability and environment-friendly nature. Apart from being studied as a potential feedstock for the synthesis of fine chemicals and generation of biofuels, these biomasses have also shown a wide range of applications in effluent treatment processes. Many agricultural waste biomasses had shown potential in effluent treatment, even in their raw form. Activated carbon prepared by the pyrolysis of biomass has yielded promising results as adsorbents and catalysts support the removal of both conventional and priority pollutants from effluents. Moreover, composite materials including metal oxide composites, magnetic materials, catalyst supports, polymer composites, and graphene composites prepared by the thermochemical conversion of biomass are being explored in tertiary treatment processes for the removal of targeted pollutants from the aqueous phase. Hence, this chapter is aimed to discuss the application of biomass-based value-added materials for effluent treatment applications.

N. Sundarabal (*) · V. Parimelazhagan Department of Chemical Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India e-mail: [email protected] S. J. Gali Anthoni Department of Chemistry, Aarupadai Veedu Institute of Technology—Vinayaka Mission Research Foundation, Rajiv Gandhi Salai, Paiyanoor, Kanchipuram, Tamil Nadu, India P. K. Ghodke Department of Chemical Engineering, National Institute of Technology Calicut, Kozhikode, Kerala, India e-mail: [email protected] S. Arumugam Catalysis Science Laboratory, CSIR-Central Leather Research Institute (CSIR-CLRI), Chennai, Tamil Nadu, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_5

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Keywords Thermochemical conversion · Biomass-based composites · Activated carbon · Effluent treatment · Emerging contaminants

Abbreviations AC BiOBr BOD COD EIA TDS

5.1

Activated carbon Bismuth oxybromide Biochemical oxygen demand Chemical oxygen demand Energy Information Administration Total suspended solids

Introduction

Humanity dates back to not a decade or a century but a millennium. Since existence began, the production of wastes in many forms originated particularly from natural sources. The most popular form of waste would be wood or of plant origin. In general, the term “Biomass is defined as matter originating from living plants, including tree stems, branches, leaves as well as residues from agricultural harvesting and processing of seeds or fruits” (Pang 2016). Biomass is considered to carry energy in the form of chemical bonds among hydrogen, carbon, and oxygen moieties (Pang 2019). The sources of biomass are plant products, the residue of crop farming and processing industries, fruit and vegetable waste, agro-industrial waste, household waste, urban waste, and animal waste (Wormeyer et al. 2011). Figure 5.1 shows the classification of biomass resources. They include materials consisting of cellulose, hemicellulose, and lignin (Mohan et al. 2006). As for the elementary composition, 90% of the typical biomass contains 51% carbon, 42% oxygen, and trace amounts of hydrogen, nitrogen, and chlorine (Mandapati and Ghodke 2021b). The biomass resources were calculated to be 146 billion tonnes/annum. Tropical countries like India contribute to the production of about 500 million metric tonnes/ year of biomass. This natural carbonaceous resource is mostly used or exploited as a source of fuel.

5.2

Biomass as a Source of Fuel

The rapid consumption of fossil fuel has led to its depletion (Mandapati and Ghodke 2021a; Agrawal and Verma 2022; Kumar and Verma 2021a). Various renewable resources which include biomass-based reserves, wind, solar, and geothermal resources have been explored as alternative fuels. Among these renewables,

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Fig. 5.1 Classification of biomass resources

bioenergy is renewable energy sourced from biomass that is abundant and has a high utilization potential to produce energy (Pang 2019; Kumar et al. 2020; Kumar and Verma 2021b). If 10% of the biomass is exploited for energy production with 50% efficiency, it may have the potential to churn out 3.1 trillion tonnes of oil equivalent energy. This would account for the availability of 200 times the energy consumed worldwide currently (Energy Information Administration, EIA, 2017). On the other hand, the utilization of 10% of biomass for organic chemical synthesis at a 10% conversion rate will lead to the production of 1.6 billion tonnes of chemical feed materials (Pang 2019). Generally, in developing countries, 38% of the energy consumed is primarily from bioenergy produced from biomasses (Sarkar and Praveen 2017). Biomass is a clean energy source is produced by plants by consuming CO2 from the atmosphere through photosynthesis (Tekin et al. 2014). Moreover, these biomasses are produced in short duration ranging from months to years when compared to unsustainable fossil fuels which usually require millions of years (Collard et al. 2012). Hence, renewable energy from biomasses proves to be a sustainable source of energy supply that can also address environmental concerns.

5.3

Value-Added Pathway from Biomass for Different Applications

Biomass conversion pathways through various processes are shown in Fig. 5.2. The widely used thermochemical conversion technologies for the conversion of biomasses include but are not limited to gasification, pyrolysis, hydrothermal

Fig. 5.2 Biomass conversion pathways

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liquefaction, and torrefaction (Goswami et al. 2020, 2021a). Torrefaction is a thermal pre-treatment process used in the combustion, pyrolysis, gasification, and liquefaction of biomass (Ghodke and Mandapati 2019). Pyrolysis is heating the biomass at elevated temperatures (573–973  K) to obtain biochar and bio-oil. The hydrothermal liquefaction process is operated between 523 and 647  K within the pressure range of 4–22 MPa for the production of bio-oil. Gasification involves the conversion of biomass into syngas at temperatures higher than 973 K.

5.4

An Alternate Strategy for the Utilization of Waste Biomass

Biomass has a great potential to be used as a source of biofuel (Bhardwaj et al. 2021). The total biomass power generation potential of India is estimated to be 17,500 MW. However, at present, only 2665 MW of power is being generated (Kumar et al. 2015). Hence, due to the availability of abundant waste biomass materials, the management or disposal of these substances is itself a major task for environmentalists. In the global arena, “Waste to Wealth” is a term coined to utilize waste material for useful resources. Since the resource is inexpensive, green, and renewable, environmentalists focus on such processes to reduce pollution. There are several studies, where the waste biomass and its derivatives were used as a precursor material for effluent treatment processes. The biomass could be converted into a base material for catalyst preparation in the form of support or as an adsorbent material. They can be utilized for the treatment of effluents by the process of adsorption or by using an advanced oxidation process (AOP). Wherein the preparation of heterogeneous catalyst by supporting the active material on carbon support derived from biomass reduces the cost of the effluent treatment process. Hence, this chapter is aimed to discuss the thermochemical conversion of biomass into value-added materials for effluent treatment applications.

5.5

Biomass for Effluent Treatment Processes

It has been reported that every day two million tons of waste from various sources are discharged into water bodies worldwide. This created an impact that one in eight people worldwide are deprived of safe and clean drinking water as reported in World Water Assessment Programme, World Water Development Report1: “Water for People, Water for Life,” Paris (2003). Water pollution is found to be the major reason for diseases and subsequently deaths worldwide (Vairavel and Murty 2020; Goswami et al. 2021b). In India alone, it has been estimated that approximately 580 people die due to water pollution-related illnesses every day (Clark et al. 1996). Major contaminants found in wastewater include but are not limited to dyes and

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pigments, heavy metals, phenolic compounds, pharmaceuticals, agrochemicals, and endocrine disruptors (Akhil et al. 2021; Kumar et al. 2019; Kumar et al. 2021). Effluent treatment plants comprise physical, chemical, and biological treatment systems. Generally, all these treatment methods are grouped under primary, secondary, tertiary, and advanced treatment processes. Primary treatment processes aim to remove contaminants by the physicochemical processes like primary clarification (gravity settling) and coagulation/flotation. The secondary treatment processes work on the removal of the residual organics. The secondary treatment processes employ various microorganisms for reducing the COD and BOD of the effluent. Activated sludge treatment methods such as aerobic and anaerobic digestion are some of the secondary wastewater treatment methods. Tertiary treatment methods include membrane separation processes, electrodialysis, advanced oxidation processes, adsorption, biosorption, bioaccumulation, and ion exchange (Sonune and Ghate 2004). Most of these treatment processes like adsorption, biosorption, and advanced oxidation processes utilize functional materials for their operation. These functional materials like adsorbents and catalysts in case of adsorption and oxidation processes respectively are of chemical origin, which leads to secondary pollution. However, the waste biomasses and their derivatives proved to be environmentally friendly precursor materials for the synthesis of adsorbents and catalysts support materials. A simple thermochemical modification of the lignocellulosic waste materials could yield highly functional materials for wastewater treatment processes (Liu et al. 2015).

5.6

Application of Raw Biomass in the Effluent Treatment Process

Adsorption is the most widely used effluent treatment technique This is because, the used adsorbent materials have the potential for regeneration, recovery, and recycling which proved to be an added advantage at the industrial scale of operations. It is not only used for the removal of contaminants but also could be used for the recovery of precious and costly entities from the effluents (Crini et al. 2019). The physicochemical properties of the adsorbent have a major influence on the efficacy of the adsorption process. The chosen or prepared adsorbent should be easily available, economical, non-toxic, and should have good surface characteristics. They must also have high mechanical and thermal stability (Reddy et al. 2017). The lignocellulosic waste biomasses, in their raw or modified form, proved to be potential candidates for the preparation of economical and sustainable adsorbents for effluent treatment (Bhatnagar et al. 2015). Plant and agricultural waste products have earned increased interest for dye and heavy metals removal by adsorption from the aqueous solution because of their natural availability and higher removal efficiency (Garg et al. 2019; Agrawal and Verma 2021). The inexpensive waste products, after basic cleaning or some minor treatment, were explored as adsorbents. Agricultural by-products

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especially those containing cellulose exhibit good adsorption potential for removing various toxic pollutants from effluents. There are numerous studies reporting the application of raw lignocellulosic materials like rice husk, oil cakes, banana peel, sugarcane bagasse, powdered leaves, etc. for the removal of different types of dyes, heavy metals, and other priority pollutants. This method of exploiting raw biomass as functional adsorbent materials proved to solve disposal problems associated with the abundant availability of these waste biomasses (Moorthy Rajendran et al. 2020). Few of the studies which deal with the application of these raw biomass materials for effluent treatment is tabulated in Table 5.1.

5.7

Biomass-Derived Activated Carbon for the Effluent Treatment Process

Many studies reported raw agricultural biomass as an adsorbent for the removal of organics and inorganics from simulated effluents. However, most of these raw biomasses were found to lack desired adsorption efficiency and were mechanically unstable. The efficiency of these biomasses could be enhanced by carbonization and activation using thermal and chemical treatment methods. The physical treatment includes carbonizing the material at a temperature of around 500  C under an inert atmosphere using nitrogen or argon supply. The carbonized material is then activated at higher temperatures using suitable activating agents like steam or CO2. The chemical treatment method involves the impregnation of the cellulosic biomass with various strong acids and bases like NaOH, KOH, HCl, H2SO4, ZnCl2, etc. The impregnated material is then carbonized at higher temperatures at around 500  C to obtain activated carbon (AC) (Rodríguez-Reinoso and Molina-Sabio 1992). The schematic for the preparation of AC through physical and chemical methods is presented in Fig. 5.3. AC is proved to have superior adsorptive and mechanical properties when compared to raw biomasses owing to increased surface area and porosity (Zhang et al. 2019). AC derived from different biomass has varying properties. The properties of AC depend on the precursor material, type of carbonization and activation, and activation temperature and duration. AC is the widely studied adsorbent material for treating effluents which contain all types of pollutants including the emerging contaminants from effluents (Yahya et al. 2015). Moreover, AC also finds its application as catalyst support in many reported studies in advanced oxidation processes which are discussed in Sect. 5.8.4. The application of biomassderived AC is not limited to effluent treatment methods, and thus they are used in various fields such as gas purification (Ma et al. 2008), supercapacitors (Yang et al. 2014), and medicinal applications (Lakshmi et al. 2018). Few of the studies on the application of biomass-derived activated carbon for water and wastewater treatment is summarized in Table 5.2.

Raw biomasses Dyes Guava leaves Coffee husks Neem leaf powder Sugar cane bagasse Almond shells Jujuba seeds Jackfruit peel Lotus leaf Heavy metals Neem leaf Cucumis melo rind Coffee residues Coconut husk Durian shell Sugarcane bagasse Cassava tuber bark waste Wheat bran Phenol Tea waste Garlic peel Rice straw Lantana camara Vegetal cords Endocrine disruptors Barley husk Walnut shell Raw fibric peat

Maximum monolayer adsorption capacity “qm” (mg/g) 93.12 38.65 39.64 38.20 22.42 55.55 285.71 45.89 157.80 4.98 39.52 2.75 117 189 83.3 101 154.39 14.48 5.78 112.5 6.21 19.94 38.5 6.48

Target pollutants

Remazol brilliant blue-R Congo red Coomassie violet Congo red Direct red 80 Congo red Methylene blue Congo red

Cd(II) Fe(II) Cd(II) Pb(II) Cr (VI) Cd(II) Zn(II) Cd(II)

Phenol Phenol Phenol Phenol Phenol

Bisphenol A Bisphenol A Bisphenol A

Table 5.1 Application of raw biomass for effluent treatment

Balarak (2016) Dovi et al. (2021) Zhou et al. (2011)

Pathak et al. (2020) Muthamilselvi et al. (2016) Sarker and Fakhruddin (2017) Girish and Murty (2014) Cherifi et al. (2009)

Sharma and Bhattacharyya (2005) Othman and Asharuddin (2013) Boonamnuayvitaya et al. (2004) Abdulrasaq and Basiru (2010) Edokpayi et al. (2015) Karnitz et al. (2007) Horsfall et al. (2006) Ozer and Pirincci (2006)

Debamita et al. (2020) Vairavel et al. (2021) Divya et al. (2020) Zhang et al. (2011a, b) Ardejani et al. (2008) Reddy et al. (2012) Hameed (2009) Meghana et al. (2020)

References

132 N. Sundarabal et al.

5

Thermochemical Conversion of Biomass into Value-Added Materials. . .

133

Fig. 5.3 Chemical and physical activation methods for the preparation of biomass-derived activated carbon

5.8

Biomass-Based Composite Materials for Effluent Treatment

Biomass-based materials either in their raw form or with thermal and chemical modification demonstrated to be an efficient low-cost material for the adsorption of various organics and inorganics from simulated effluents. Furthermore, recent studies suggest that these biomasses can also be fused with other functional materials and could be actively applied in various treatment processes. These biomass-based composites include but are not limited to metal oxide composites, magnetic materials, polymeric materials, and graphene-based composites. The schematic representation of the same is presented in Fig. 5.4.

5.8.1

Biomass-Based Magnetic Materials for Effluent Treatment

Generally, nanoparticles have promising potential to be used in various effluent treatment technologies. However, the application of these nanoparticles in water and wastewater purification is limited. This is due to the fact that, the spent nanomaterials after application tends to escape into the aquatic environment causing secondary pollution. The removal and recovery of the spent materials in their nanoform is not economical. Recently, materials coated with magnetite nanoparticles are considered

Amido black 10B

Pb(II) Pb(II) Ni(II)

Cd(II)

Palm flower

Heavy metals Dairy waste Sugar beet Hazelnut shell

Canna indica

Apple pulp and peel

Acid yellow 36 Reactive orange Methylene blue, Metanil yellow Methylene blue

Rice husk Sugarcane bagasse pith Grape processing waste

–

–

–

–

– – –

–

–

873 873 –

steam – –

373 – –

–

–

Methylene blue

Pineapple waste

–

–

Direct blue 106

873

– – 423

413

HCl

– – H2SO4

H2SO4

– H3PO4 ZnCl2 ZnCl2 H3PO4

– 873 873 –

ZnCl2

ZnCl2, H3PO4, H2SO4

Chemicals used

773

393

773

–

–

Target pollutants

Direct navy blue 106

Chemical Temperature (K)

Orange peel

Biomass-derived activated carbon Dyes Pomegranate peel

Activation of adsorbents Physical Temperature Activating (K) agent

Table 5.2 Biomass-derived activated carbon for effluent treatment

189

248 197 11.64

4.03

86.9 3.48 417 386 278

288.34

107.53

54.05

Maximum monolayer adsorption capacity “qm” (mg/g)

Inyang et al. (2012) Demirbas et al. (2002) Cui et al. (2016)

Amin (Amin 2009) Khaled et al. (Khaled et al. 2009) Mahamad et al. (2015) Malik (2003) Amin (2008) Saygili et al. (2015) Hesas et al. (2013) Nethaji and Sivasamy (2011)

References

134 N. Sundarabal et al.

Cd(II)

Phenol

Phenol

Phenol

Phenol

Parachlorophenol Phenol Para-nitrophenol

Pine bark Oak bark Peanut shell

Corn stalk

Phenols Bamboo charcoal

Banyan root

Date-pit

Rattan saw dust

Dates’ stones

Sulfamethoxazole

Sulfamethazine

Amoxicillin

Pharmaceutical wastes Pine sawdust

Tea waste

Giant reed

Corn husk

Cu(II) Zn(II) Pb(II) Pb(II) Pb(II)

Corn straw

973

973

Microwave

Steam

–

– –

– – –

–

–

CO2

–

– 1173

–

–

–

473

– – – – –

– – 723 723 –

–

–

–

FeCl2, KOH, KNO3 –

FeCl3 FeCl3

ZnCl2

923

773 773

773

KOH

–

– 973

KOH

–

– – – – H3PO4, HNO3 ZnCl2

773

–

873

873 873 – – 823

10

`19.09

8.445 11.668

102.04

149.25

262.3

26.95

24.96

32.4

12.5 11.0 3.0 13.1 35.5

Thermochemical Conversion of Biomass into Value-Added Materials. . . (continued)

Rajapaksha et al. (2016) Chayid and Ahmecd (2015)

Reguyal and Sarmah (2018)

Ma et al. (2013) Nirmala et al. (2019) El-Naas et al. (2010) Hameed and Rahman (2008) Aldoury and Sabeeh (2014) Mishra et al. (2019)

Chen et al. (2011) Mohan et al. (2007) Xu and Liu (2008) Youssef et al. (2004)

5 135

– –

–

1073

452

KOH

Methanol

HNO3

–

17α-ethynyl estradiol Tetracycline

Montmorillonite/rice husk hydrochar Sawdust hydrochar

493

–

–

Caffeine

Pistachio nut shells

–

–

–

573

Bisphenol A

–

–

–

Argun nut shells

–

–

HCl

–

973

–

–

573

Ketoprofen

Wheat straw

–

–

–

773

Diuron

Chlortetracycline

Cauliflower roots

H3PO4

723

–

–

1173

Diclofenac

Moringa seed

H3PO4

773

–

–

Tetracycline

Paracetamol

Olive stones

Chemicals used KOH

Chemical Temperature (K) 773

Activation of adsorbents Physical Temperature Activating (K) agent – –

Endocrine disruptors Agave Americana leaf fibers in combination with tannin Argun nut shells

Target pollutants Oxytetracycline

Biomass-derived activated carbon Cassava waste

Table 5.2 (continued)

423.7

138

22.6

1162.79

833.33

87.21

72.46

81.30

121.112

108.3

Maximum monolayer adsorption capacity “qm” (mg/g) 3.33

Zbair et al. (2020) Zbair et al. (2020) Roman et al. (2018) Tian et al. (2018) Chen et al. (2017)

Selmi et al. (2018)

References Luo et al. (2018) Garcia-Mateos et al. (2015) Bagheri et al. (2020) Qin et al. (2017) Wu et al. (2018)

136 N. Sundarabal et al.

Pramipexole Dorzolamide

Tetracycline

Potato peel hydrochar

Rice straw hydrochar

– – –

– – – 573

473 473 K2CO3

KOH KOH 714

66 60

Kyzas and Deliyanni (2015) Liu et al. (2014)

5 Thermochemical Conversion of Biomass into Value-Added Materials. . . 137

138

N. Sundarabal et al.

Fig. 5.4 Biomass-based composites for effluent treatment

very promising in the environmental remediation process. Since the magnetic particles are superparamagnetic (they are magnetized only with the external magnetic field), they can be recovered easily by the external magnetic field and reused without any loss of the functional materials (Nethaji et al. 2013). Hence, magnetic materials like iron oxide nanoparticles are extensively studied as functional materials for their application in water treatment owing to their magnetic properties. However, these iron oxide nanoparticles tend to agglomerate in the solution, thereby decreasing their efficiency. Hence, biomass-based materials supported by iron oxide nanocomposites were studied and successfully applied to overcome these drawbacks (Mehta et al. 2015). Biomass-based magnetic composites are mainly used in the adsorption process, and there are few studies which deals on the application of these materials in photocatalytic oxidation (Pang et al. 2016). Though several methods like hydrothermal reactions, sol-gel methods were reported, the co-precipitation method is most commonly employed for the preparation of magnetic composites. In the co-precipitation method, the biomass, or biomass-derived materials like activated carbon (AC) are dispersed along with the iron precursors like ferric chloride or ferrous sulfate. The iron salt in the precursors is then reduced using various reducing agents under continuous stirring, thus depositing the iron oxide nanoparticles onto the matrix of the raw biomass or AC derived from the biomass (Nethaji et al. 2013). The efficiency of these magnetic composites expended for the adsorptive/oxidative treatment of various pollutants from the aqueous phase along with the experimental conditions are shown in Table 5.3. The application of magnetized adsorbents derived from raw biomass and AC is presented in Fig. 5.5. Most of the studies reported an enhanced removal efficiency of the magnetic nanocomposites in comparison with the unmagnetized materials. Few adsorption studies reported a slight decrease in the efficiency, owing to the reduction in the available surface area due to the impregnation of iron oxide particles into the matrix of the biomass. Nevertheless, the ease of separation of the functional material improved considerably, thereby aiding in the regeneration and reusability of the nanocomposite materials.

Peanut shell

Magnetic activated carbon from peanut shell

722.34 (CO2 activation) 448.70 (Without CO2 activation)

280.39

Acid Blue 113

Palm Kernel Shell (PKS)

Magnetic graphene oxide-biomass activated carbon composite

Malachite Green

153.89 (MCA) 69.45 (unmagnetized carbon)

Surface area (m2/g)

Methylene Blue

Target pollutants

Corn cob

Biomass-based magnetic adsorbents Dyes Magnetic Corn cob-derived carbon (MCA)

Precursor biomass materials

Table 5.3 Biomass-based magnetic composites for effluent treatment

747.03 (CO2 activation) 270.28 (Without CO2 activation)

32.2

163.93 (MCA) 103.09 (unmagnetized carbon)

Maximum monolayer adsorption capacity “qm” (mg/g) The efficiency of the magnetized and unmagnetized adsorbent derived from corn cob was compared. The magnetized carbon possessed better surface area and adsorption efficiency for the sorption of methylene blue dye. The adsorbent had the saturation magnetization value of 33.74 emu/g as characterized by a Vibrating Sample Magnetometer. A comparison study was also carried out to prove that the magnetized graphene oxidePKS-derived carbon composites had better removal efficiency than the raw precursor and graphene oxide. Studies were performed by increasing the iron oxide content in the matrix of the carbon derived from the peanut shell. The surface area, porosity, and adsorption efficiency increased with the increase in the iron oxide impregnation proving that the degree of magnetization is one of the most important parameters.

Inferences

Thermochemical Conversion of Biomass into Value-Added Materials. . . (continued)

Guo et al. (2018)

Ying et al. (2020)

Ma et al. (Ma et al. 2015)

References

5 139

Emerging contaminants Fe3O4/Douglas fir Douglas biochar fir

Baker’s yeast

EDTAD-modified magnetic baker’s yeast biomass (EMB)

Caffeine (stimulant) Ibuprofen (Inflammatory drug) Acetylsalicylic

Lead (II)/cadmium (II)

Nickel (II)

Tea waste

Magnetic nanoparticle (Fe3O4) impregnated onto tea waste

Target pollutants

Chromium (VI)

Precursor biomass materials

Phoenix tree leaves

Biomass-based magnetic adsorbents Heavy metals Magnetic biochar composites (MB)

Table 5.3 (continued)

468.2 (Douglas fir biochar) 322.0 (Fe3O4/ Douglas fir biochar)

–

22.3 (tea waste) 27.5 (magnetized tea waste)

83.6

Surface area (m2/g)

Caffeine: 23.9 (Douglas fir biochar) 73.1 (Fe3O4/Douglas fir biochar) Ibuprofen: 15.5 (Douglas fir

89.21 (Pb2+) 41 (Cd2+)

55 (Magnetized Biochar) 39.8 (Unmagnetized Biochar) 26.5 (Fe3O4 nanoparticles 38.3

Maximum monolayer adsorption capacity “qm” (mg/g)

It was observed that the unmagnetized adsorbent exhibited better surface area when compared to the magnetized Douglas fir biochar. However, the adsorption of all the three pharmaceuticals were better

Panneerselvam et al. (2011)

This study does not involve the preparation of activated carbon from tea waste. Tea waste is directly incorporated with Fe3O4 nanoparticles. The incorporation of iron oxide nanoparticles had a negligible impact on the surface area. EMB could be regenerated using both HCl and EDTA with an efficiency of greater than 90% without disturbing the morphological characteristics.

Liyanage et al. (2020)

Zhang et al. (2011a, 2011b)

Liang et al. (Liang et al. 2019)

References

Magnetized biochar was prepared by hydrothermal method. The O2 containing groups on the surface of the biochar provided growth sites for Fe3O4 nanoparticles.

Inferences

140 N. Sundarabal et al.

acid (Inflammatory drug)

biochar) 32 (Fe3O4/Douglas fir biochar) Acetylsalicylic acid: 89 (Douglas fir biochar) 126 (Fe3O4/Douglas fir biochar)

using the magnetized adsorbent compared to the unmagnetized char.

5 Thermochemical Conversion of Biomass into Value-Added Materials. . . 141

142

N. Sundarabal et al.

Fig. 5.5 Biomass-derived magnetic composites for effluent treatment

5.8.2

Biomass-Based Polymer and Clay Composites for Effluent Treatment

The efficiency of a functional material in the effluent treatment mainly depends upon its available surface area and surface functionality for adsorptive removal, and oxidative potential in case of advanced oxidation processes. Hence, polymeric materials are potential candidates to be used in the adsorption process, owing to their tunable surface properties (Pan et al. 2009). Naturally occurring biopolymers such as starch, cellulose, chitosan, and alginate were widely explored for their adsorption potential due to their high surface area and variable surface functionality. Among all the above-mentioned biopolymers, chitosan is widely explored for the removal of contaminants from simulated wastewater. It is more preferred since it contains -NH2 and -OH functional groups on its surface which can act as chelating sites for concentrating organic and inorganic moieties (Wang and Zuang 2018). However, most of these naturally occurring biopolymers exhibit weak thermal and mechanical properties. Moreover, these biopolymers exhibit swelling phenomena when exposed to the aqueous environment. Hence, most of the studies deal with crosslinking these biopolymers with mechanically strong materials like biochar, biomass-derived AC, or clay composites. Natural clay materials like bentonite, montmorillonite, kaolinite, zeolite, etc. are mostly aluminosilicates with the presence of sodium, potassium, magnesium, and calcium. The layered morphology of these clay materials with the charged surface is ideal for the adsorption of ionic contaminants from the effluent. However, these clay minerals have poor potential for the removal of non-ionic contaminants. Nevertheless, they possess strong mechanical and thermal stability. Hence, the biopolymers are generally cross-linked with clay minerals to overcome the shortcomings of both clay and biopolymers (Unuabonah and Taubert 2014). Hence, biopolymers supported with biochar, AC, and clay minerals exhibited superior adsorption efficiency with better thermal and mechanical stability as shown in Table 5.4.

Chitosan from shrimp shell, activated carbon from grape stalks

Chitosan

Chitosan/activated carbon/iron bio-nanocomposite

Zerovalent iron encapsulated chitosan nanospheres (CIN)

Biopolymer clay composites Cellulose-montmo- Cellulose rillonite composite

B. subtilis var. (natto)

Bacteria-derived poly(γ-glutamic acid) (γ-PGA)

Biomass-based adsorbent Biomass precursors Biopolymer composites Zeolite derived Chitin from chitin

Chromium (VI)

Arsenic (V) and (III)

Cadmium

Crystal violet, methylene blue, basic fuchsin Auramine O, rhodamine B, safranin O

Target pollutants

87.09

834 (activated carbon) 11.6 (chitosan) 419.20 (chitosan/ activated carbon/ iron bio-nanocomposite) 69 (CIN) 26 (zerovalent iron)

–

–

Surface area (m2/g)

Table 5.4 Biomass-based polymer and clay composites for effluent treatment

22.2

–

124 (Crystal violet), 87.45 (methylene blue), 789.10 (basic fuchsin) 0.05 dm3/mg (auramine O), 0.02 dm3/mg (rhodamine B), 0.19 dm3/ mg (safranin O) 322.58

Maximum monolayer adsorption capacity “qm” (mg/g)

The interaction between cellulose and montmorillonite has proved the potential application

The study shows that the adsorbent is porous in nature and iron particles exist in zerovalent state and there exists a complexation reaction among iron, chitosan, and arsenic

The study indicated better interactions between oxygen functional groups of AC, iron ions, and amine groups of chitosan

The zeolite was synthesized using biopolymer chitin as a mesoporosity agent by the hydrothermal method The kinetic data were well in agreement with Boyd’s ion-exchange model

Inference

Thermochemical Conversion of Biomass into Value-Added Materials. . . (continued)

Kumar et al. (2011)

Gupta et al. (2012)

Sharififard et al. (2018)

Inbaraj et al. (2006)

Briao et al. (Briao et al. 2018)

References

5 143

Alginate

Guar gum

Guar gum /bentonite bio-nanocomposites

Biomass precursors

Montmorillonitealginate beads

Biomass-based adsorbent

Table 5.4 (continued)

Lead (II), crystal violet (CV) dye

Paraquat herbicides

Target pollutants

5.533

46

Surface area (m2/g)

187.08 (Pb2+) 167.92 (CV)

0.321 mmol/g

Maximum monolayer adsorption capacity “qm” (mg/g) of the material for the effective adsorption of chromium. The XRD peaks signified the ordered distribution of clay layers in the biopolymer composite TGA was used to determine the thermal stability of the beads. The results proved that the thermal stability of montmorillonite-alginate beads was much better than alginate beads The TGA results proved increased thermal stability of biocomposites in comparison with guar gum. FT-IR suggested the electrostatic interaction or chelation via hydrogen bond formation between Pb2+ or CV and active sites of bio-nanocomposites.

Inference

Ahmad and Mirza (2018)

Etcheverry et al. (2017)

References

144 N. Sundarabal et al.

5

Thermochemical Conversion of Biomass into Value-Added Materials. . .

5.8.3

145

Biomass-Based Graphene Composites for Effluent Treatment

Graphene is composed of single layers of carbon atoms densely packed which attracted tremendous attraction in late 2004. It is a “single layer of carbon atom densely packed in a honeycomb crystal lattice” (Li et al. 2019). Because of its good chemical stability and graphitized basal plane structure, graphene-based materials are widely used in different applications including supercapacitors, fuel cells, batteries, and as adsorbents in effluent treatment systems (Novoselov et al. 2012). Graphene or reduced graphene oxide is mostly synthesized by using Hummers or modified Hummers method. Various functional groups present on the edges of the graphitic planes aid in the interaction of the graphene sheets with the charged contaminants present in the wastewater. However, the graphene sheets generally suffer from stacking and agglomeration problems due to π–π interactions and van der Waals forces in the aqueous phase. Hence, to overcome these limitations and to exploit the desirable properties of the graphene-based materials, various biomassbased graphene composite materials were reported for the removal of organic and inorganic compounds from simulated effluents (Nethaji and Sivasamy 2017). Biomass-derived materials like biochar, AC, and other cellulosic waste biomass were used as composite materials by crosslinking with the honeycomb structure of graphene oxides. There are also studies which had reported on the utilization of iron oxide nanomaterials for acquiring the magnetic properties reducing the stacking problem of graphene layers. The application of these biomass-based graphene composite materials for the removal of organic and inorganic moieties is presented in Table 5.5.

5.8.4

Biomass-Based Metal Oxides Composites such as Catalyst and Catalyst Supports

Heterogeneous catalysis involving metal oxides is a good example of an advanced oxidation process. Literature shows a number of methods for the enhancement of catalyst activity alongside cost reduction and efficiency maximization. Though many paths are sorted for catalyst modification, synthesis of a catalyst supported on materials with a higher surface area without diminishing the activity is a greater concern. Hence, the choice of the support material comes into the picture, wherein it must be cheap, green, and environmentally friendly with enhanced activity. Hence, carbon as catalyst support derived from biomass is a better option. Wherein, the disposal problem of the biomass itself is minimized and the resulting carbon could be used efficiently. However, activated carbon in itself has been used as a catalyst. Juhola et al. (2021) had prepared biomass–metakaolin as granular composite materials for application as a catalyst for the treatment of effluents.

Lead (II)

Phenol

Glucose

Lentinus edodes

Graphene/activated carbon composite

Nitrogen-containing carbon nano-onions-like and graphenelike materials

Rhodamine B

Rice husk

Graphene oxide/silica nanocomposites

Target pollutants Methylene blue

Biomass precursors Rice husk

Biomass-based graphene composites Graphene oxide-based carbonaceous nanocomposites

35.40

2012

625

Surface area (m2/g) 936

Table 5.5 Biomass-based graphene composites for effluent treatment

–

217.6

147.06

Maximum monolayer adsorption capacity “qm” (mg/g) 1591 Inference Graphene oxide/ordered mesoporous carbon was prepared from rice husk using mesoporous silica as a template source Silica was extracted from rice husk. The hybrid material was prepared by hydrothermal method. The study also claims that the composites not only aid in recycling agricultural waste but also help in the recovery of graphene oxide from the aqueous phase Mesoporous graphene/activated carbon composite was prepared from graphene oxide and glucose. The prepared composite had a higher adsorption capacity for Pb2+ when compared to the reported literature Nitrogen-containing carbon materials with different morphologies were prepared using Fe(NO3)3 through a hydrothermal method. The prepared material was used for both adsorption and photocatalytic oxidation of phenol. The photoelectrons transformed by graphite claim to be reacted with O2 molecules to form the superoxide radicals (˙O2) for the degradation of phenol

Han et al. (2021)

Saeidi et al. (2015)

References Liou and Wang (2020) Liou et al. (2021)

146 N. Sundarabal et al.

5

Thermochemical Conversion of Biomass into Value-Added Materials. . .

147

Since the development of nano metal oxides as a photocatalyst under visible light irradiation for environmental remediation (Fujishima and Honda 1972). Various research has been focused on the modification of metal oxides for maximum efficiency. The preparation of metal oxide on catalyst support is highly researched. While a choice for support material activated carbon prepared from biomass is widely preferred. Devagi and Soon (2018) have reported a TiO2/modified sago bark (biomass) for treating the sago wastewater effluent. The author chose the effluent as it had a higher chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TDS), and was acidic. The sago bark was chosen as a precursor as it is a major waste during the debarking step of the starch extraction. The modified biomass TiO2 mixture was used as a photocatalyst for 64.92% removal with 0.2 g/L TiO2/1% MSB (120 min of irradiation). Poudel et al. (2020) has removed As(III) from water by using agro-waste-based biomass impregnated with TiO2. ZnO incorporation on biomass-derived activated carbon has been widely researched by various research groups. Cruz et al. (2018) had prepared a ZnO/activated carbon (biomass derives) nanocomposite for the treatment of methylene blue dye. The ZnO nanoparticles were evenly distributed on the surface of the activated carbon. Ramya et al. (2018) has worked on the preparation of activated carbon from tannery sludge biomass. The acquired biomass was used as support material for ZnO-based nanocomposite preparation. The material was used for Cr (VI) removal from the aqueous phase. Supported biomass-based activated carbon for dye degradation has been prepared by hydrothermal technique. Waste biomass was used by Vinayagam et al. (2018) for activated carbon preparation as carbon support. Akpomie et al. (2020) recently prepared a ZnO nanoparticle along with biomass for the treatment of celestine blue dye. Hybrid bifunctional materials have also been synthesized by various research groups comprising AC and nanomaterials which would serve both as an adsorbent and photocatalytic material. Our group has also worked in this area (Nethaji et al. 2018) and we have reported a bismuth oxybromide (BiOBr)/activated carbon hybrid material as a bifunctional nanomaterial for effluent treatment. It was a good adsorbent material and it even degraded malachite green dye under visible light irradiation. The source of AC was waste polyurethane foam trash from used car seats. The composition of the foam was a polyol with a toluene diisocyanate blend. A simple hydrothermal process was used for the preparation of the bifunctional material.

5.9

Conclusions

Biomass materials are mostly explored for their potential to be used as biofuel. However, the consumption of this biomass for the production of biofuel accounts for less than 10% of the available lignocellulosic materials. Owing to its abundant availability, most of the agricultural waste biomass is considered waste and requires a separate disposal method or a process for the same. But the utilization of these

148

N. Sundarabal et al.

types of wastes and naturally available renewable material as a source for the production of materials for the effluent treatment itself is a boon to the environment. Most of the studies suggest that these biomasses either in their raw form or modified form proved to be an effective replacement for the synthetic materials conventionally used in effluent treatment operations. Moreover, the hybrid materials which include biomass-based composites could effectively overcome the shortcomings of their synthetic counterparts. The biomass-derived activated carbon was effectively used in various adsorptive and catalytic applications as adsorbent and catalyst support, respectively. The compatible and environmentally friendly nature of the biomasses could be explored for applications in various other unit operations in effluent treatment plants. Hence, these materials derived from the biomasses can be coined as “waste to wealth” and thus prove to be environment-friendly substitutes to treat and overcome effluent treatment problems. Competing Interests All the authors declare that they have no competing interests.

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Chapter 6

Cellulase: A Catalytic Powerhouse for Lignocellulosic Waste Valorisation Sukhendra Singh , Ipsita Chakravarty Jyoti Srivastava, and Rupika Sinha

, Shankar Mukundrao Khade

,

Abstract Cellulose is the world’s most ubiquitous organic compound and has an abundant potential to be transformed into a renewable source of energy and other industrial products. To break down this complex polymer, cellulase has been conventionally used as a biocatalyst. Other than numerous industrial uses, including detergent, paper and pulp, textile, beverages, feed, baking, and biofuel, the application of cellulase for waste valorisation is also a central thrust area. The cellulolytic action of this enzyme has been explored for utilisation of widely available lignocellulosic waste. This reaction has been critical for the cellulose conversion to glucose and then to biofuel. Apart from biofuel, cellulase has also been employed to convert green waste into value-added products such as prebiotic oligosaccharides, organic acids, and biopolymers that can be integrated into the circular economy. Numerous factors affect the catalytic actions of cellulase, like pretreatment of lignocellulosic biomass, structural and compositional variation, working temperature, and pH. This chapter encompasses a detailed insight into the biocatalytic mechanism, applications, and limitations of cellulase as a potent enzyme for waste valorisation. Keywords Cellulase · Waste valorisation · Lignocellulosic biomass · Circular economy · Biorefinery

S. Singh Department of Biotechnology, Dr B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India I. Chakravarty Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India S. M. Khade School of Engineering, Ajeenkya D Y Patil University, Pune, Maharashtra, India J. Srivastava · R. Sinha (*) Department of Biotechnology, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_6

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Abbreviations 3Rs ATCC BSG CBG CMCase CNCs CNF CO2 Co60 FPA FPU GHG IFPU IL IU LCA MNPs MPa PHA PLA RSM SDS SHF SmF SMS SmSF SSCF SSF TCA TEA U/g U/mL USD

6.1

Reduce reuse and recycle American type culture collection Brewer’s spent grain Compressed biogas Carboxymethyl cellulase Cellulose nanocrystals Cellulose nanofibrils Carbon dioxide Cobalt-60 Filter paper assay Filter paper units Greenhouse gas International unit of filter paper activity Ionic liquid International enzyme unit Life cycle assessment Magnetic nano-particles Megapascal Polyhydroxyalkanoates Poly-lactic acid Response surface methodology Sodium dodecyl sulphate Separate hydrolysis and fermentation Submerged fermentation Spent mushroom substance Simultaneous Saccharification and Fermentation Saccharification and co-fermentation Solid-state fermentation Tricarboxylic acid Techno-economic analysis Enzyme unit per gram Enzyme unit per millilitre United States dollar

Introduction

The focus of waste treatment has now shifted to its valorisation, as it promises better management of available resources. The valorisation method, if assisted by biological techniques, becomes a better environment-friendly solution. The utilisation of biological methods for waste valorisation has spawned the concept of a bio-based

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circular economy. Cellulase is one such biocatalytic agent which is categorised as endoglucanase (E.C.3.2.1.4), exoglucanase, or cellobiohydrolase (E.C.3.2.1.91), and β-glucosidase (E.C.3.2.1.21). The source of cellulase can vary from natural to commercial production (Bhardwaj et al. 2021a). The commonly reported bacteria, streptomyces, and fungi for cellulase production are Clostridia sp., Bacillus licheniformis, Streptomyces argenteolus, S. griseorubens, S. lividans, Aspergillus oryzae, Candida cylindracea, Trichoderma reesei, etc. (Hamdi et al. 2020; López-Mondéjar et al. 2019; Saldarriaga-Hernández et al. 2020; Kumar et al. 2018, 2020a). The genetically developed producers include E. coli and Pichia pastoris. The mechanistic insight of cellulase explains the hydrolysis process. Endoglucanase is responsible for the breakdown of β-1,4 glycosidic bond, which exposes new chain ends. The ends so produced are broken by exoglucanase/ cellobiohydrolase to produce oligosaccharides, whereas β-glucosidase causes lysis of these oligosaccharides into their monomer units. For a detailed review of the mechanism of action and structure of cellulase, an article by Rabinovich et al. can be referred (Rabinovich et al. 2002). Most of the cellulases have been found to be a multidomain proteins containing a catalytic domain, a cellulose-binding domain and an interdomain linker. Earlier reports suggest the use of cellulase in waste management such as deinking of paper, degradation of voluminous cotton sludge, and bioconversion of sawdust, vegetable and fruits waste, rice straw, leaves and bamboo, and sawdust. Thus, cellulase-mediated waste valorisation becomes a potent tool for the proficient utilisation of agro-residues, food, paper, and textile industry waste (Khan et al. 2016; Vermelho et al. 2012). The cellulose content of municipal solid waste has also been utilised for biogas production (Demirbas et al. 2016). According to a report, about 10–50 billion tons of dry lignocellulosic waste are produced every year across the globe (Motaung and Linganiso 2018). This waste has been reported to contain about 45% cellulose, 10% hemicellulose, and 10–15% lignin. The agrowaste mainly includes sugarcane bagasse (grown in China, India, Brazil, Thailand, South Africa, and Australia), maize stalks (major crop of countries like South Africa and the United States), rice husk (grown worldwide), and sorghum stalks (tropical and subtropical regions of Asia and Africa). According to Khan et al., lignocellulosic material consists of 30% solid content composed of about 65% holocellulose and 20% lignin (Khan et al. 2016; Kumar and Verma 2021a). The meticulous design of the valorisation method can lead to the recycling of by-products with economic advantage. With the advancement of current technologies, diverse applications of these processes have been explored. These include fossil-fuel alternatives such as bioethanol, biopropanol, aviation fuel, and biogas (Kumar and Verma 2021b). Other platform chemicals, including succinic acid, butyric acid, and enzymes are also being produced by using enzymatic hydrolysis of food and agro-industrial waste. This further extends to the industrial production of innumerable polymers with various commercial applications such as polyhydroxy butyrate, poly-lactic acid, xanthan, pullan, and poly-hydroxy valerate. The application of cellulase for the valorisation of waste from different industries has been summarised in Table 6.1.

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Table 6.1 Application of cellulase in the waste valorisation Value-added product formed Bioethanol

Industrial waste as substrate Lignocellulosic biomass

Source of cellulase Bacillus. tequilensis G9

Forest waste (spruce and birch)

Obtained from NZYTech Lda.

Prebiotics

Paper sludge

Obtained from Novozymes and Sigma-Aldrich

Bioethanol production

Lignocellulosic biomass

yeast Yarrowia lipolytica

Sugarcane bagasse

Bacterial consortium from sugarcane bagasse and thermophilic sludge A. niger

Lipid (ricinoleic acid) production Methane, hydrogen, and other organic acids

Spent mushroom and food waste

Lactic acid

Textile waste (cotton-polyester 50:50 blend)

Commercial cellulase and β-glucosidase

Bioplastics, biosurfactants, and other biochemicals

Tapioca flour commercial waste Pomegranate peel

NA

Bioethanol gel

Commercial cellulase and pectinase (Novozyme)

Antimicrobial property for application in therapeutics

Method used Co-culturing of Bacillus. tequilensis G9 with Saccharomyces cerevisiae Pretreated residues were enzymatically digested by four cellulase Supplementation of β-glucosidase with a commercial cocktail of enzyme Consolidated Bioprocessing

Reference Dar et al. (2019)

Karnaouri et al. (2019)

Gomes et al. (2016)

Guo et al. (2018)

Bioaugmentation of cellulase producing bacteria obtained from lignocellulosic biomass with sludge

Soares et al. (2019)

Cellulase produced by A. niger led to the breakdown of waste to produce hexoses and pentoses. These were then utilised by Enterococcus mundtii CGMCC 22,227 Pretreatment, enzymatic hydrolysis, and recovery step include the use of activated carbon, filtration. Ion exchange chromatography and solvent evaporation Pretreatment, enzymatic hydrolysis, and fermentation High-pressure extraction at 300 and 600 MPa

Ma et al. (2021)

Subramanian et al. (2020)

Amalia et al. (2021) Alexandre et al. (2019)

(continued)

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Table 6.1 (continued) Industrial waste as substrate Sugarcane straw

Sugarbeet

Source of cellulase Commercial enzyme (Celluclast and Novozyme 188) Accellerase (Merck-Sigma Aldrich)

Value-added product formed Bioethanol

Pectin, phenolic compounds, and succinic acids

Method used Steam explosion pretreatment followed by enzymatic hydrolysis Fractionation is followed by enzymatic hydrolysis. The hydrolysate was fermented by Actinobacillus succinogenes to produce succinic acid

Reference Oliveira et al. (2013)

Alexandri et al. (2019)

This chapter encompasses applications of cellulases in the valorisation of various waste belonging to food, textile, agricultural, and other industrial sectors. The current state of the art pushes toward a bio-based circular economy where the waste stream is maximally recycled to other production input stream.

6.2

Recent Advances in Cellulase Production and Purification

The potential of abundant natural polymer cellulose in nature has been critically known in biofuel production such as bioethanol and biobutanol. However, the major constraint is in the conversion of cellulose to fermentable sugars (saccharification) that can be overcome by treating the biomass with the cellulase enzyme. The cellulolytic enzyme can be obtained from plant, animal, and microbial sources (Zhang and Zhang 2013), among which microbial sources can be used to scale up production.

6.2.1

Microbial Production of Cellulase

Most cellulolytic microorganisms were isolated from the decaying wood and used to produce the cellulase enzyme at a large scale by either solid-state fermentations or submerged fermentations.

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Solid-State Fermentation (SSF)

The type of fermentation occurs almost without free water; however, the substrate moisture must be present to assist the microorganism’s growth. Hence, SSF imitates the natural habitat of maximum number of cellulolytic filamentous fungi. The critical factors that play a crucial role in the extracellular release of cellulase in SSF are moisture content, harvest time, spore count, particle size, type of substrate, incubation time, temperature, and initial pH values that render the adaptation cellulase excretion. The SSF begins with inoculating an optimised number of pure spores of filamentous fungus into the sterile solid medium under the necessary optimised conditions. The cellulase production using Trichoderma reesei ZU-02 through SSF by reusing solid substrates in at least three batches yielded the optimum cellulase production (158 IFPU/g koji) was obtained in the second batch of fermentation (Xia and Cen 1999). The comparative investigation of cellulase production in SSF by the two mutant strains of Trichoderma reesei showed to be critically varied with the physicochemical parameters like moisture content and the temperature in which T. reesei QM9414 strain indicated that only moisture content (70% optimum) had a major influence on the production of cellulase with maximum FPA (filter paper assay) activity (1.1635 U/g). Whereas, temperature (25
C optimum) and moisture (55% optimum) were found to be important in the extracellular cellulase production with maximum FPA activity (2.314 U/g) by T. reesei MCG77 strain (Latifian et al. 2007). In addition, Kim et al. had investigated the improvement of cellulase production by Aspergillus sp. SU-M15 with the repeated sequential treatment of mutagens Co60 γ-rays, N-methyl-N0 -nitro-N-nitrosoguanidine, and ultraviolet irradiation to spores of Aspergillus sp. The production in wheat bran medium was found to be higher (82.5 U/g) than husk and sawdust by SSF (Vu et al. 2011). Sometimes, a mixture of substrates induces the extracellular cellulase production rather than the individual substrate, as investigated by Barnabe et al., who found that the mixed sludge of paper and pulp that influenced the lignocellulolytic secretions to 7.3 IU/mL from 1.5 IU/mL by Trichoderma reesei RUT C-30 (Lai et al. 2017). However, the cost of cellulase production is considerably reduced by utilising the waste residues or low-cost substrates from the municipal corporation, horticulture, kitchen, industrial, or open terrestrial sources as the substrates for SSF by optimising the required physicochemical environment (Bansal et al. 2012; Bharti et al. 2018; Xin and Geng 2010). The purification of extracellular cellulase enzyme can be done by extraction with the circulation of buffer and collection as a crude cellulase preparation that can be used for the saccharification of lignocellulosic materials and other applications.

6.2.1.2

Submerged Fermentation (SmF)

The type of fermentation that occurs in the presence of excess water is known as submerged fermentation (Kumar et al. 2020b). In most industrial-scale enzyme

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operations, the production by SmF is of priority because of ease of monitoring and operating. Most of the commercially available cellulases are produced by filamentous fungi; however, some bacteria (Ariffin et al. 2006; Ekperigin 2007; Irfan et al. 2012) and actinomycetes (Das et al. 2014; George et al. 2001; Van Zyl 1985) can also have the ability to produce but in low titre. The aerobic production by SmF depends on several factors like cellulosic substrate, nutrient availability, pH of the medium, inducer concentration, rate of oxygen supply, and fermentation temperature. The media formulation is necessary to result in optimal growth of biomass and cellulase production. The generalised medium used for the growth of filamentous fungi and cellulase enzyme production is Mandel’s medium, Vogel’s medium. However, to increase cellulase production, these medium needs to be optimised with respect to cellulolytic fungus. The cellulase production can be increased by genetically engineering the cellulolytic strains by either random mutation or sitedirected mutagenesis. Separation of extracellular enzymes from the fermented solid biomass is carried out by centrifugation (10,000  g, 10 min, 4
C) that can be used directly as a crude enzyme for the hydrolysis of cellulosic material (Lai et al. 2017). The cellulase production by SSF and SmF using various substrates has been listed in Table 6.2.

6.3

Immobilisation and Molecular Approaches for Enhanced Cellulolytic Action

The objectives of immobilisation of cellulase enzyme are reusability of the same enzyme for several cycles with excellent efficiency and stability that can substantially reduce the overall cost of the product. The materials used to immobilise the enzymes are both organic and inorganic, including polysaccharides, proteins, polymers, activated carbon, and metals (Datta et al. 2013; Nawaz et al. 2016; Safarik et al. 2016). There are various strategies to immobilise the enzymes, as represented in Fig. 6.1. Recent reports suggest several methods for cellulase immobilisation for the valorisation of agro-residues, as shown in the above figure (Fig. 6.1). There have been studies for the comparison of conversion efficiency of free and immobilised enzymes for bioethanol production using lignocellulosic residues. The use of nanostructured support for the immobilisation of cellulase has been widely investigated (Ahmad and Khare 2018; Brindhadevi et al. 2020; Han et al. 2018; Husain 2016; Sillu and Agnihotri 2019). These nano-carriers impart reusability and stability of the enzyme and exhibit a larger surface area for enzyme attachment. This facilitates overcoming the mass transfer limitations by increasing the amount of enzyme that can be employed for catalytic conversion. The use of Magnetic Nano-Particles (MNPs) further improvises the process by allowing easy withdrawal of enzymes from the solution utilising an external magnetic field. In another study, the immobilisation was done on MNPs produced using green synthesis. Immobilisation

Melanocarpus sp. MTCC 3922 Penicillium echinulatum 9A02S1 Trichoderma reesei Rut C30 Trichoderma atroviride 676

Penicillium citrinum

Rice straw Pretreated sugarcane bagasse Pretreated willow Sugarcane bagasse

Wheat bran

1.72 U/mL

SmF SmF SSF

0.62 FPU/mL 0.25 U/mL

SSF SSF

17.8 U/mL

0.396 U/mL

Activity FPU 1.14 U/mL

3.49 U/g of substrate 25.3 U/mL 32.89 U gdm-1

Acremonium cellulolyticus strain CF2612 Fomitopsis sp. RCK2010 SSF

SmF

Trichoderma spp.

Enriched Mandel’s cellulose medium Solka Floc

Wheat bran

SmF

Microorganism Trichoderma reesei RUT C30

Substrate Wheat bran

Fermentation process SSF

Table 6.2 Examples of cellulase production using different substrates

1.89 U/mL

132 U/mL 1.9 U/mL

–

0.07 U/mL 0.17

53.6 U/g of substrate 74.1 U/mL 58.95 U gdm-1

40.3 U/mL

– 71.6 U/g of substrate 42.3 U/mL 282.36 U gdm-1

–

β-glucosidase 0.22 U/mL

0.488 U/mL

CMCase assay 14.98 U/mL

Jatinder et al. (2006) Camassola and Dillon (2007) Kovács et al. (2008) Grigorevski-Lima et al. (2013) Dutta et al. (2008)

Deswal et al. (2011)

Fang et al. (2009)

Reference Sukumaran et al. (2009) Chand et al. (2005)

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Fig. 6.1 Various methods to adopt immobilisation of enzymes

was secured by the covalent linking of cellulase to MNPs using glutaraldehyde. The results showed higher conversion efficiency of free enzymes (78%) than the covalently bonded enzymes (72%) in the first cycle, owing to the mass transfer limitation. However, subsequent cycles showed stable conversion efficiency, which ultimately is responsible for improved process economics (Ingle et al. 2017). Thus, immobilisation of cellulase contributes to its reusability, stability, and storage. Cellulase can also be covalently immobilised using glutaraldehyde as a linker. In a significant work, it was immobilised on biochar prepared using sugarcane bagasse and coated with chitosan in different concentrations. This immobilisation process was successful in achieving 90% residual activity of cellulase even after 10 cycles of conversion (Mo et al. 2020). A detailed analysis of the pros and cons of selecting suitable micro- and nano-carriers with different methods of immobilisation is available elsewhere (Rajnish et al. 2021).

6.4

Role of Cellulase in Biorefinery Application

Cellulases are potential candidates for the bioconversion of agro-industrial wastes into several beneficial products. Such wastes are otherwise burnt or used in landfills (Singh et al. 2021). A zero-waste integrated biorefinery would involve cellulases in

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Fig. 6.2 Application of cellulases in a lignocellulosic biorefinery

the sequential production of sugar, organic acids, surfactants, furfurals, and biofuels such as bioethanol, biobutanol, biohydrogen, and compressed biogas (CBG) using agro-industrial residues, and the remaining organic matter can be used as compost or animal feed (Carrillo-Nieves et al. 2020; Kumar and Verma 2021a, b). Commercial enzyme manufacturing companies strive to manufacture enzyme cocktails for industrial biorefineries (Chandel et al. 2012). The biorefinery scheme utilising cellulases is depicted in Fig. 6.2.

6.4.1

Consolidated Bioprocessing for Biorefinery

The integrated approach to biomass conversion is known as consolidated bioprocessing. In this, a single microbe’s amalgamation of saccharification and fermentation capabilities reduces environmental losses and expenses in lignocellulosic biorefineries (Bhardwaj et al. 2021a; b). The cellulase production, enzymatic hydrolysis, and bioconversion are combined (Cunha et al. 2020). The method involves hydrolysis of the structural carbohydrates to oligomers using the synthesised enzymes. The oligomers are depolymerised. The residual sugar is then fermented into various valuable products (Daniel et al. 2012). 10–30% of the pretreated liquor contributes as the carbon source for the production of enzymes and propagation of seeds. The enzyme loading is reduced to 10 mg protein/g cellulose (Scarlata et al. 2015). These modifications make the process economical and efficient. In a recent study on the biorefinery approach, consolidated bioprocessing using engineered Trichoderma reesei and Saccharomyces cerevisiae led to the highest production of 0.5 g/L glucaric acid from steam-treated corn stover in 7 days (Li et al. 2021). In another study, consolidated bioprocessing was used to produce bioethanol using surfactant (Tween 20)-assisted ionic liquid (1-ethyl-3methylimidazolium methane sulphonate) from pretreated biomass of Parthenium

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hysterophorus L. The optimisation of pretreated conditions like biomass loading, pretreatment temperature, and time could increase the sugar yield by 40% during saccharification with IL-stable cellulase and xylanase enzymes produced from Aspergillus aculeatus PN14 (Nargotra et al. 2019). A novel consolidated bioprocessing method was developed for hyperproduction of butyric acid from delignified rice straw using co-fermentation of Clostridium thermocellum ATCC 27405 and C. thermobutyricum ATCC 49875. The butyric acid yield was increased up to 33.9 g/L by fed-batch fermentation with a selectivity of 78% (Chi et al. 2018). Therefore, consolidated bioprocessing combines many processes and reduces the overall cost of production by reducing energy requirements and contamination risk as well as maximising cost-effectiveness.

6.4.2

Cellulases in Agro-industrial Waste Biorefinery

Enzymatic pretreatment is an indispensable step of any lignocellulosic biorefinery. Using cellulolytic hydrolysis, the waste from agricultural practices and industries can be converted into several valuable products (Dragone et al. 2020). A cellulolytic enzyme derived from Bacillus pumilus is used for sugar production from bananaagro waste in the form of pseudostem and leaves (Kanmani et al. 2011). Cellulases synthesised by Sporotrichum sp. LAR5 with 7.88 IU was used to extract reducing sugar from acid-pretreated rice straw (Bajaj et al. 2014). In another instance, cellulases derived from Penicillium decumbens were used to hydrolysis corn cobs to produce reducing sugar used for ethanol production (Saliu and Sani 2012). The use of cellulases enzyme for the valorisation of various agricultural and industrial wastes is depicted in Table 6.3. Cotton waste from textile industries is decomposed using cellulases for safe disposal into the environment. Waste generated from textile industries includes cotton and denim residues from the cotton machining process. These waste streams are treated with cellulase. Such industrial residues are subjected to enzymatic hydrolysis for recycling and production of valuable products such as glucose syrup (Subramanian et al. 2020). Fruit and vegetable wastes are rich in disaccharides and polysaccharides. They are also non-competitive with our food chain. Cellulases can hydrolyse the fruit and vegetable waste for further bioconversion into numerous commercial products (Srivastava et al. 2021). Hemp Hurd is a by-product of the fibre industry. Immobilised cellulase on an activated magnetic support was used to hydrolyse hemp hurds (Abraham et al. 2016). In another study, Liu et al. (2019) investigated the hydrolysis of apple pomace by pectinase and cellulase and further optimised the cellulase hydrolysis parameters by RSM after pectin hydrolysis. They obtained a maximum fermentable sugar production yield of 67.54  1.45%, which provided the highest lipid production of 25.8 g L1 after fermentation by a genetically modified strain Yarrowia lipolytica po1f (pex10mfe leu+) (Liu et al. 2019).

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Table 6.3 Application of cellulases for treating different agro-industrial biorefinery Agroindustrial waste Paper sludge

Newspaper waste

Enzyme source Recombinant cellulase cocktail, Opt CelMix similar to Cellic CTec2 and Celluclast Celluclast 1.5 L and Novozyme 188

Citrus peel waste

Indigenously from Aspergillus niger

Corn stover

Commercial acidic cellulase Trichoderma reesei and Aspergillus niger Indigenously from Trichoderma harzianum

Sugarcane bagasse Wastewater sludge

Rice husk

Celluclast

Textile residues

Indigenously from Aspergillus niger CKB

Sorghum waste

Spezyme-CP and Novozyme

Processing parameters 0.015 g/g cellulase cocktail and 10% (w/v) biomass loaded

Pretreatment using SDS and Tween-80. 15% solid content and enzyme loading was 15 FPU/g waste

30 IU g-1 Drm cellulase, processing at 116
C for 10 min Steam-exploded 20 FPU/g dry stover 2.0 IU/g cellulases

Bubble column bioreactor and membrane reactor for a system with 30 kDa membrane Lime pretreated biomass, 48 FPU/g Alkali pretreated biomass, 0.43  0.01 FPU g enzyme loading 10% pretreated (by ammonia) fibres were hydrolysed using 60 FPU Spezyme-CP and 64 CBU Novozyme 188/g glucan

Observations 80% glucan conversion. High glucose yields using low enzyme loading

References Malgas et al. (2020)

29.07 g/L reducing sugar produced after 72 h at 50
C. 0.42 g/g ethanol produced by co-culture of Saccharomyces cerevisiae and Pichia stipitis 11.18 mg/ L reducing sugar. 30.7 g/ L ethanol and (339–356 mL/gVS methane) 103 g/L reducing sugar 0.5 g reducing sugar obtained per dry substrate 73.5% cellulase recovery and 81.37% reduction in chemical oxygen demand

Xin et al. (2010)

0.2 g/g reducing sugar

70.2% reducing sugar recovered

84% digested for a pretreated sample and 38% cellulose digested for untreated biomass 0.24 g ethanol per g dry biomass was obtained for the pretreated sample

Patsalou et al. (2019) Lu et al. (2010) Rabelo et al. (2008) Libardi et al. (2019)

Saha and Cotta (2008) Hu et al. (2018)

Salvi et al. 2010

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Challenges of Cellulase-Based Biorefinery

Driving down the cost of enzyme production poses the main challenge for agroindustrial biorefineries. Mass bio-prospecting programme for the discovery of efficient cellulase enzyme synthesising microbes can make the production economical in the long run (Jayasekara and Ratnayake 2019). Integrated processing techniques would also be helpful in improving cellulase production and ethanol productivity in a simplified manner (Kuhad et al. 2016). Instead of single enzymes, concoctions and cellulolytic enzyme cocktails should be explored for their applications in hydrolysis (Victoria et al. 2017). In-house enzyme production is an economical option to produce ethanol for biorefineries (Cunha et al. 2017). This would ensure waste to wealth conversion in a sustainable manner.

6.5

Role of Cellulase in Value-Added Product Formation

Lignocellulosic biomass can be transformed into value-added chemicals through various existing chemical, thermochemical, and biological routes. However, the biological method of lignocellulosic biomass conversion using enzymes is a feasible and environment-friendly, and sustainable process. Many bacterial and fungal species have the ability to breakdown the lignocellulosic biomass into simple sugars such as glucose and xylose by secreting enzymes (cellulase and hemicellulase), which can further be utilised to produce several value-added products like acids (lactic acid, citric acids, succinic acid, gluconic acids), butanol, xylitol, microbial polysaccharides, and other fine chemicals (Ning et al. 2021). In combination with other enzymes (utilised pectinase and xylanase), cellulase improves the hydrolysis efficiency and reduces the overall cost associated with the process (Bhardwaj and Verma, 2021). Some of the important value-added products derived from lignocellulosic biomass are mentioned in Table 6.4.

6.5.1

Lactic Acid

Lactic acid is used as a green solvent and building block for polylactic acid (PLA), a bio-degradable plastic alternative to petroleum-based plastic. Lactic acid can be produced from simple carbohydrates derived from the hydrolysis of agricultural lignocellulosic biomass. However, these recalcitrant lignocellulosic materials should be pretreated first to ease the cellulosic hydrolysis process governed by the cellulase enzymes. Various pretreatment such as physical (milling, grinding, shredding), chemical (acidic, alkali, ionic liquids), and thermal pretreatments can enhance the cellulosic hydrolysis process based on the nature of the lignocellulosic biomass. Yadav et al. (2021) inspected the role of imidazolium ionic liquid in the pretreatment

Fruit and Vegetable waste Paper mill sludge Microcrystalline cellulose Brewer’s spent grain

Succinic acid

Butyric acid Gluconic acid PHA

Substrate used Rice straw Corn stover biomass

Product Lactic acid Citric acid

9.0  0.44 mg PHA/g dry BSG

–

Liu et al. (2018) Ruales-Salcedo et al. (2020) Llimós et al. (2020).

Burkholderia cepacia

Li et al. (2018)

0.46 gg-1 – 0.27 gg-1

Reference Yadav et al. (2021) Hou and Bao (2018)

Yield – 74.9% of cellulose

7.0 9.60

43.1

Concentration (g/L) 36.75 136.3

Clostridium tyrobutyricum –

Microorganism L. plantarum SKL-22 Aspergillus niger SIIM M288 Yarrowia lipolytica

Table 6.4 List of some value-added chemicals produced from lignocellulosic biomass

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of rice straw and further saccharified with commercial cellulase enzyme for lactic acid production by Lactobacillus plantarum SKL-22 strain. They observed that the media optimisation using response surface methodology could enhance the lactic acid yield by 1.11%, with a final yield of 36.75% using rice straw in a single pot bioprocess. In many bioprocesses, utilised cellulase enzymes are used repeatedly to saccharify lignocellulosic biomass to simple carbohydrates, which could be further used to produce value-added chemicals. This could be a holistic approach for upcoming biorefineries in the future and sustainable product formation. Grewal et al. (2018) explored the one-pot bioprocess of lactic acid production from ionic pretreated lignocellulosic waste saccharified by nano-immobilised cellulase enzyme. In this process, the simultaneous saccharification and co-fermentation (SSCF) process was performed by Lactobacillus brevis, which provided a lactic acid yield of 0.22, 0.52, and 0.49 g/g with sugars derived from ionic pretreated cottonseed cake, sugarcane bagasse, and wheat straw, respectively. To reduce a large amount of cellulase requirement in lignocellulosic-based lactic acid production, Chen et al. (2020) established the integration of fed-batch simultaneous saccharification process with membrane separation, in which residual cellulase enzyme present in the waste stream and solid-residue of corn stover were recycled and reused for successive fermentation. Around six rounds of operation were carried out, which increased the lactic acid yield (0.389 g/g) by 1.2 times higher than the conventional process and reduced the cellulase consumption, wastewater discharge and nutrient consumption by 47%, 73.7% and 86.1%, respectively. In addition to lignocellulosic biomass, other agricultural wastes such as food waste and fungal biomass have also been utilised to produce lactic acid with the assistance of the cellulase enzyme. Ma et al. (2021) studied the co-fermentation of spent mushroom substance (SMS) and food waste to produce lactic acid. They reported that 92.62% of sugar was released from pretreated SMS by Aspergillus niger cellulase, which could increase the lactic acid concentration by 22.97% (Ma et al. 2021). In another study, Abdel Rahman et al. (2019) isolated a thermo-alkaliphilic lactic acid bacterium, Enterococcus faecium FW26, which could produce lactic acid from mixed food waste and banana peels with a lactic acid yield of 84% g/g-consumed sugars. Enterococcus faecium FW26 secreted the amylase and cellulase enzyme and grew on a mixed substrate of food waste and banana peels without any treatment. In addition, the isolated strain was able to survive and facilitate fermentation under the stress of thermo-alkaline conditions, which might reduce the chances of contamination.

6.5.2

Succinic Acid

Succinic acid is a dicarboxylic acid and an intermediate compound of the tricarboxylic acid (TCA) cycle. It has various uses in the food industry as a flavouring agent,

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acidulant, and antimicrobial agent and is also used as a solvent additive, surfactant, foaming agent, detergent extender, coating resins, and ion chelator (Kumar et al. 2019). Glucose, maltose fructose, sucrose, and glycerol have been the most frequently used carbon sources for succinic acid production by fermentation using strains of bacteria such as Anaerobiospirillum succiniciproducens, Actinobacillus succinogenes, Mannheimia succiniciproducens, and recombinant Escherichia coli (Karthik and Rathinamoorthy 2017). However, utilisation of cheaper carbon sources such as lignocellulosic biomass with the help of cellulase enzyme could improve the overall process economics. Some microorganisms can utilise partially hydrolysed cellulose and produce valuable products. Jiang et al. (2013) performed the incomplete hydrolysis of cellulose for succinic acid production by Actinobacillus succinogens. A succinic acid concentration of 20 g/L with a yield of 64.7% was achieved from 18 g/L of cellobiose and 17 g/L of other sugars obtained from sugarcane bagasse hydrolysate. In another study, Li et al. (2018) hydrolysed the fruit and vegetable waste with a cocktail of enzymes (2% glucoamylase, 2% hemicellulase, 1% cellulase, and 0.25% pectinase) and obtained the final glucose titre of 56. 7 g L1 and a yield of 0.46 g g1. The obtained hydrolysate was further utilised to produce succinic acid by Y. lipolytica PSA02004 with a titre and yield of 43.1 g L1 and 0.46 g g1, respectively. They improved succinic acid productivity (0.69 g L1 h1 ) and concentration (140.6 g L1) using in situ fibrous bed bioreactor and fed-batch fermentation. Therefore, cellulase facilitates the utilisation of agro-waste for the production of value-added chemicals through biochemical pathways with good efficiency and cost-effectiveness.

6.5.3

Gluconic Acid

Gluconic acid, a non-toxic, non-corrosive, and weak organic acid is industrially produced through the fermentation of glucose by A. niger or Gluconobacter suboxydans. The gluconic acid production was also reported by various strains of Pseudomonas, Acetobacter, and Zymomonas. Gluconic acid has many applications in the food industry as a flavouring agent, in dietary supplements as different salts (sodium, calcium, and iron gluconate), and in the pharmaceutical and textile industries. Microbial fermentation is the most suitable and chosen method for gluconic acid production due to being less expensive and more efficient than other methods. Large-scale gluconic acid production is mainly affected by three crucial factors: cheaper substrate, oxygen concentration, and pH. Using the lignocellulosic biomass can solve the problem of the cheaper substrate if a cost-effective enzyme production method is developed. Yu et al. (2021) explored the co-immobilisation of multienzymes (cellulase, catalase, and glucose oxidase) on reversibly soluble polymer Eudragit L-100 in a cascade reaction for the direct transformation of corn straw to gluconic acid. The gluconic acid yield was achieved at 0.28 mg/mg, and the rate of conversion of cellulose in corn straw to gluconic acid was obtained at 61.41%. Similarly, Zhou et al. (2019) studied an integrated process of co-production of

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gluconic acid with xylooligosaccharides from sugarcane bagasse. After the xylooligosaccharides production from the prehydrolysis, 88.6% of cellulose was converted into simple sugars in fed-batch enzymatic hydrolysis using cellulase (C2730, Celluclast®). The gluconic acid yield of 96.3% was produced by fermentation of sugars using Gluconobacter oxydans ATCC 621H. Therefore, approximately 105 g of xylooligosaccharides and 340 g of gluconic acid were co-produced in the integrated process from 1 kg of dried sugarcane bagasse (Zhou and Xu 2019).

6.5.4

Citric Acid

Citric acid is another crucial organic acid used as a preservative and acidulant in the food industries and also used as a starting material in the pharmaceutical and cosmetic industries. It is mainly produced through microbial fermentation from using carbohydrates (mainly starch) or agro-industrial waste. However, due to the restricted supply of food starch, potential alternative sources like lignocellulosic biomass are also being explored to meet future demand. Cellulase plays a vital role in the conversion of these recalcitrant compounds into simple sugars. Citric acid is produced on large scale through fermentation by A. niger due to its high citric acid productivity and tolerance to low pH. It is mainly produced through submerged fermentation. However, many researchers have explored the use of agro-residues and lignocellulosic biomass as the starting material for citric acid production. Hou et al. (2018) studied the simultaneous saccharification and fermentation (SmSF) of dilute acid-pretreated corn stover for citric acid production by A. niger. In SmSF, pretreated corn stover was hydrolysed by cellulase enzyme, Cellic CTec 2.0, from Novozymes. The highest citric acid concentration (136.3 g/L) was obtained with an overall yield of 74.9% of cellulose in the aerobic SmSF process. This study compared the citric acid production from SmSF with separate hydrolysis and fermentation (SHF) and concluded that the cellulosic citric acid production by SmSF could be highly comparable to starch-based citric acid production.

6.5.5

Microbial Polymers

6.5.5.1

Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHA) are the most potential, degradable, and environmentfriendly substitute for petrochemical-based plastics. The main constraint that prevents PHA from commercialisation is the cost of production. Therefore, researchers are focusing on cheap substrates for sustainable PHA production. In this regard, lignocellulosic waste can be used as a possible substrate as it is cheap, readily available, and most importantly, it does not compete with the food requirements of humans. Nowadays, a wide range of lignocellulosic waste generated from industries,

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forestry, agriculture, marine biomass, etc. has gained great consideration for PHA production (Obruca et al. 2015). PHA production using lignocellulosic biomass at the industrial level includes mainly three steps, especially pretreatment, enzymatic/acidic hydrolysis, and fermentation. Many pretreatment techniques are used before the enzymatic hydrolysis of cellulose which includes physical processes for size reduction, chemical treatment (with hydrogen peroxide, acid, alkali, or sodium chlorite) for delignification, physicochemical processes (like a steam explosion, ammonia fibre expansion, etc.) or a biological pretreatment using microbial enzymes. Further, enzymatic hydrolysis is performed for the degradation of complex carbohydrates into simple sugar, which serves as a carbon source for microbes in the successive fermentation step. Later fermentation is carried out using PHA-producing microbes (e.g., Bacillus sonorensis, Halomonas hydrothermalis, Pseudomonas putida, Bacillus cereus, etc.) and PHA is recovered. It was suggested that the average sugar and PHA yields were higher in the case of enzymatic hydrolysis as compared to acid-based hydrolysis and cellulase plays an important role in enzymatic hydrolysis (Al-Battashi et al. 2019). Production of PHA without pretreatment steps is also used to decrease the cost of the process. Israni and Shivakumar (2020) used cellulase, lipase, and amylase positive Bacillus megaterium strain Ti3 for the production of PHA without pretreatment step. PHA was produced using 16 different lignocellulosic substrates by submerged fermentation at 30
C for 48 h at 120 rpm. They demonstrated that ragi husk and sesame oil cake have the most influential factors on the basis of Taguchi orthogonal array. Similarly, Heng et al. (2017) demonstrated a three-step process for PHA production using Burkholderia cepacia USM, Cupriavidus necator NSDG-GG, and a genetically modified strain of Cupriavidus necator H16. They used rice husk as raw material and performed alkaline pretreatment followed by enzymatic hydrolysis and fermentation. For enzymatic hydrolysis, they used a cellulase-based enzymes Celluclast 1.5 L and Novozyme-188 at 50
C 160 rpm for 72 h. They obtained the highest sugar yield of 87% after enzymatic hydrolysis and obtained 7.8 g/L of dry cell weight and 50% PHA content using a 5 L fermentor. Llimós et al. (2020) performed two-stage valorisation using Brewer’s spent grain (BSG) as a lignocellulosic substrate for PHA production. First, they produced lignocellulolytic enzymes by three fungal strains (Trichoderma reesei, Aspergillus niger and Thermoascus aurantiacus) through SSF of dried BSG. Then these enzyme were used for the hydrolysis of BSG and production of PHA. The PHA was produced by fermentation of sugar rich BSG hydrolysate at 30
C and 120 rpm using Burkholderia cepacia and Cupriavidus necator. C. necator cells were harvested after 48 h and B. cepacia cells were harvested after 72 h. The maximum yield of PHA they obtained was 9.0  0.44 mg PHA/g dry BSG, and the result showed that BSG could be utilised as a suitable raw material for the production of value-added products (Llimós et al. 2020). PHA is a potential material with a broad range of uses which includes its use as bioplastics, drug carrier, and synthesis precursor, packaging materials, biofuels, implant materials, cosmetic ingredients, disposal cups, and bottles, etc. Presently,

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PHA is not well commercialised as compared to fuel-based plastics and products. The main hurdles are the high cost of production and relatively poor characteristics. Still, researchers are searching for the solution to this problem as PHA offers an eco-friendly and sustainable alternative to petrochemical-based products and has a promising future (Wang and Chen 2017).

6.5.5.2

Nanocellulose

In the last few years, remarkable development has been noticed in nanotechnology, especially in nanocellulose production. Nanocellulose produced from lignocellulosic waste is a potential alternative for obtaining various value-added products that can meet the demand of the market and environmental issues (Jaiswal et al. 2021). According to various literature, lignocellulosic biomass like rice husks, wheat straw, cotton, sugarcane bagasse, flax fibres, wood bark, softwood pulp, corncob, hardwood, etc. can be used for the production of nanocellulose (Michelin et al. 2020). Currently, acid hydrolysis is the most regular method for the production of nanocellulose. However, nowadays production of nanocellulose using enzymatic hydrolysis is gaining significant importance as toxic residues are not produced. Moreover, during enzymatic hydrolysis, mild conditions are used, which makes it a low energy-intensive process (Karim et al. 2017). The process of nanocellulose production by enzymatic hydrolysis contains several steps. The first or pretreatment step consists of delignification where lignin is removed using various mechanical processes (chipping and milling) and chemical processes (alkaline pretreatment and bleaching pretreatment). The second step is enzymatic hydrolysis which is generally carried out at the pH range from 4 to 7 and temperature range from 45 to 50
C. The last step includes homogenisation, where the enzymatically treated material is homogenised by washing or mechanical homogenisation (Ribeiro et al. 2019). de Aguiar et al. performed enzymatic hydrolysis of sugarcane bagasse and straw at 50
C and 200 rpm. The cellulose nanocrystals (CNC) produced by both bagasse and straw showed a crystallinity index of approximately 70% and high thermal stability (de Aguiar et al. 2020). In another study, Yarbrough et al. (2017) reported that the synergistic effect of two or more enzymes shows a better effect than an individual enzyme. They demonstrated that the Caldicellulosiruptor bescii with multifunctional enzymes is better in cellulose and nanocellulose production than the classical system of “free enzyme” produced by Trichoderma reesei. The application of only one technique is not as effective as the combination of two or more techniques. For example., high-quality nanocellulose is produced using a coupled ball milling process and enzymatic hydrolysis, which can be further improved when combined with any other technique (Teo and Wahab 2020). Zhang et al. (2020) designed a new approach using poplar wood as lignocellulosic biomass to produce nanocellulose. They used steam explosion and delignification for the pretreatment of poplar wood. Further, the enzymatic hydrolysis was performed using cellulase enzyme followed by ultrasonic treatment. The optimum

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condition determined by them for enzymatic hydrolysis was 200 U/g at 50
C for 12h, giving approximately 13.2% yield of nanocellulose. The crystallinity of the composite produced in this study was 61.98% which was higher than the crystallinity of poplar cellulose, and the structure of the composite was not damaged during the preparation steps. Squinca et al. (2020) performed enzymatic hydrolysis using a cellulolytic enzyme complex with the high specific activity of endoglucanase produced by the fungus Aspergillus niger. They pretreated the cellulose pulp of eucalyptus by ball milling, followed by enzymatic hydrolysis and sonication, which yielded 24.6% of cellulose nanocrystals (CNCs) by the enzymatic hydrolysis of 96 h. CNC’s length, diameter, and crystallinity were 294.0 nm, 24.0 nm, and 78.3%, respectively. Apart from nanocellulose isolation, cellulases can also be used for the pretreatment of lignocellulosic waste and other pretreatment techniques. Zeng et al. (2020) used endocellulase for enzymatic pretreatment of the bleached pulp of softwood kraft before wet ball milling. Further, they used mechanical grinding for the production of cellulose nanofibrils (CNF). Nanocelluloses have a wide range of applications in different industries it can be used as material for food packaging in food industries, electronic displays in the electronic industry, paper and coating material in the paper and pulp industry, a supporting material in the medical and tissue industry, absorbent material in the water treatment industry, etc. (Zaki et al. 2021). The enzymatic hydrolysis is a timeconsuming process for the production of nanocellulose. However, it has gained the attention of companies and researchers as it has shown better saccharification efficiency and penetration. Despite the challenges, enzymatic hydrolysis for nanocellulose production has great potential because of its unique characteristic and eco-friendly nature (Teo and Wahab 2020).

6.6

Role of Cellulase in a Circular Economy

The application of cellulases for the efficient valorisation of agro-industrial waste has tremendous potential for generating biofuels and biochemicals with low carbon and water footprints that can be recycled back to the earth within the scope of a circular economy (Astolfi et al. 2019). This conceptualises the 3Rs of a circular economic system. In situ resource utilisation, enzyme recycling, and sequential zerowaste biorefinery approaches are commercially viable solutions for sustainable development.

6.6.1

Valorisation via Carbon-Neutral Technologies

Fossil-based resources are now being replaced by alternative renewable feedstocks for the synthesis of cellulases and bioconversion for biofuels, commercially

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important chemicals, and biomaterials to envisage a decarbonisation system (Adsul et al. 2020). Millions of tonnes of lignocellulosic waste are generated per annum globally. Their valorisation in an economical and eco-friendly manner forms the core of a bio-based circular economy (Sheldon 2020). In the current scenario, the entire focus lies in the thoughtful conversion of the so-called wastes into valuable products such as reducing sugars, biochemicals, and biofuels, primarily regulated by cellulase enzymes.

6.6.2

Life Cycle Assessment of Cellulase-Based Biorefinery

The environmental and economic effects of a process analysed by the life cycle assessment (LCA) technique include all inputs and outputs of material and energy flow at different life cycle stages (Russell et al. 2005; Kumar and Verma 2021c). LCA technology in the assessment of cellulase production and utilisation is limited by rigid system boundaries, data accuracy and availability, variations in statistical techniques, variations in product utility, its selectivity, and local environmental conditions (Borrion et al. 2012). System boundaries of the cellulase manufacturing process focus on the greenhouse gas emissions related to the cradle-to-gate activities for cellulase production (Fig. 6.3). In a case study on cellulase production from corn feedstock, Aspen-Plus and GREET 1.8c.25 were used for estimation of emissions and financial matrices. 258 g CO2 eq. L–1 of bioethanol was estimated for onsite production and 403 g CO2 eq. L–1 for offsite production. The cost estimation was made on the basis of bioethanol plant size and cellulase loading. An additional input like return on investment is used for offsite data. The final production cost was 0.46 USD gal–1 of bioethanol for cellulase for on-location production (Hong et al. 2013). Cellulase production from coffee husk was assessed on the basis of data taken from EcoInvent v3.4 (SimaPro v8.5 software) database. The results showed that the downstream process significantly impacts the environment and economic value (Catalán et al. 2019). In another study, an integrated ethanol production process using spruce loggings was compared to an offsite production scenario. The greenhouse gas emissions were lower in the onsite process as compared with that of the offsite process. The ethanol selling rate was 0.568 euro per litre for the integrated cases and 0.581 euro per litre for the off-location case (Olofsson et al. 2017).

6.6.3

Increased Circularity via Onsite Application

The change in the substrate from enzyme production to hydrolysis and offsite enzyme application often alters the enzymatic efficiency. Researchers have found that the microorganisms grown on various carbon sources under different physicochemical environments produce different isoforms of cellulase, which show behavioural diversity while interacting with the same substrate (Siqueira et al.

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Fig. 6.3 System boundaries for cellulase application in biorefinery

2020; Zhao et al. 2018). The techno-economic feasibility was assessed for the onsite and offsite cellulase synthesis and utilisation for ethanol generation using corn stover as feedstock. The biomass channelised from the ethanol production stream improved the value of onsite cellulase production (Kazi et al. 2010). The comparative routes for onsite, offsite, and integrated biorefinery are shown in Fig. 6.4.

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Fig. 6.4 Comparison of offsite, onsite, and integrated biorefinery

Novozyme introduced Cellic CTec3 based on the economic bioconversion of pretreated lignocellulosic substrate (Sun et al. 2015). The techno-economic model for the corn-based ethanol industry revealed the significance of onsite cellulase production for biorefinery. The use of pure glucose, priced $580/t as a carbon source, covers over 50% of cellulases’ production cost, which is commercially not feasible (Humbird et al. 2011). Working on a low-cost substrate, non-sophisticated downstream processes, less storage, and transport makes onsite enzyme production eco-friendly and contributes to a circular economy.

6.6.4

Challenges and Perspectives

The circular economic sustainability is challenged by two major threats. These are the lignocellulose biorefinery supply chain and the resistant structure of lignin, which restrict the access of cellulose by cellulases (Garlapati et al. 2020). The last issue significantly affects the cellulose crystallinity and the efficacy of cellulases. Low-cost, energy-efficient pretreatment strategies can resolve the lignin problem. The energy recovery process needs to be accompanied by soil replenishment (Battaglia et al. 2021). In this context, it is essential to re-conceptualise the enzymatic reduction of agricultural lignocellulosic waste for obtaining value-added products through sustainable utilisations. The integrated lignocellulosic biorefinery leads to the circular economic framework through the endorsement of niche markets,

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consumer-oriented value generation, and reduction of transportation through supply chain modifications of lignocellulosic feedstocks (Sarsaiya et al. 2019). The ultimate benefits would be a cleaner environment and economic profitability.

6.7

Conclusion

Cellulase plays an important role as it is responsible for the hydrolysis of cellulose into simple sugars, which can be further used as a carbon source for microbial fermentation. Nowadays, valorisation of lignocellulosic waste has gained the attention of researchers, and efforts are made to make this process industrially and commercially feasible as it is eco-friendly and has great potential and a promising future. Cellulases are produced at a large scale using cellulolytic microorganisms either by SSF or SmF. However, industries prefer SmF over SSF as it is easy to monitor and operate. The commonly reported microorganisms for cellulase production are T. reesei, S. argenteolus, B. licheniformis, Candida cylindracea, Aspergillus oryzae, etc. Cellulases are a powerful tool for the bioconversion of agro-residues, food, paper, and textile industry waste into beneficial products like surfactants, furfurals, aromatics, phenols, ethanol, butanol, lactic acid, succinic acid, gluconic acid, citric acid, etc. Recently, the use of cellulase is extensively explored for the production of microbial polymers such as nanocellulose, polyhydroxyalkanoates, polyhydroxy butyrate, xanthan, and pullan. The major challenge behind the use of lignocellulosic biomass in biorefineries is the cost of production. Many strategies have been developed to reduce the cost such as the utilisation of immobilisation techniques for the reusability of cellulase, onsite cellulase production, use of enzyme cocktails instead of a single enzyme, combination of two or more techniques for the pretreatment step, and use of low-cost substrates. Despite many challenges, great progress has been made in the direction of using lignocellulosic biomass as it is a potential renewable alternative for fossilbased resources. Also, advanced tools such as techno-economical analysis (TEA) and life cycle assessment analysis (LCA) provide insight into actual benefits associated with enzyme-based conversion processes over chemical processes. These assessments promise an optimised circular economy approach for maximum fulfilment of societal needs and minimum impact on the environment and ecosystem. Competing Interests There is no conflict of interest.

References Abdel-Rahman MA, Hassan SE-D, Roushdy MM, Azab MS, Gaber MA (2019) Free-nutrient supply and thermo-alkaline conditions for direct lactic acid production from mixed lignocellulosic and food waste materials. Bioresour Technol Rep 7:100256 Abraham RE, Wong CS, Puri M (2016) Enrichment of cellulosic waste hemp (Cannabis sativa) hurd into non-toxic microfibres. Materials 9(7):562

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Chapter 7

Technical Criteria for Converting Biomass to High Liquid Bio-Oil Yields Naval Koralkar and Praveen Kumar Ghodke

Abstract Rising energy demands and depletion of fossil fuels have led the research community to investigate alternative fuel sources. Green and renewable biofuels have evolved to substitute for a non-renewable energy source. Biomass can be utilized as a raw material for producing low-carbon fuels. Although biomass-based fuels can replace fossil fuels, direct use is limited due to the low quality of the fuels and expensive process costs. An unrivaled solution to this problem is an integrated biorefinery concept involving generating hydrocarbon-grade fuels and valuable chemicals from pyrolysis-derived bio-oil. The chapter examines recent breakthroughs in bio-oil up-gradation processes and moisture removal techniques and bio-oil recovery of valuable compounds. One of the widely used and well-developed techniques for producing bio-oil is the fast pyrolysis of biomass. The catalytic cracking process has been identified as a viable technology for converting bio-crude to liquid fuel in bio-oil upgrading. The chapter examines recent trends and advances in the fast pyrolysis technique to improve overall profitability of the process. Critical analysis of the potential and existing techniques and necessary future steps are essential for adopting these methods industrially and in a feasible manner. Keywords Pyrolysis · Upgraded fuel · Green fuel · Biomass

N. Koralkar Department of Chemical Engineering, ITM (SLS) Baroda University, Vodadora, Gujarat, India P. K. Ghodke (*) Department of Chemical Engineering, National Institute of Technology Calicut, Kozhikode, Kerala, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_7

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Abbreviations CHNS-O CO2 DSC SEM FTIR NMR NOx SO2 TGA XRD

7.1

Analyzer Carbon dioxide Differential scanning calorimetry Scanning electron microscope; Py-GC/MSchromatography/mass spectrometry Fourier transforms infrared spectroscopic analyzer Nuclear magnetic resonance spectroscopy Nitrogen oxides Sulfur dioxide (SO2) Thermogravimetric analysis analyzer X-ray diffraction analyzer

Pyrolysis-gas

Introduction

Biomass is a promising eco-friendly alternative renewable energy source in today’s energy landscape. The present global energy supply is primarily dependent on fossil fuels such as oil, natural gas, and coal, with limited reservoirs (Smith et al. 2018; Goswami et al. 2021a). Thus, it has become necessary to evaluate an alternative long-term energy source that can meet the demands of the growing global population and the increasing per capita energy consumption. Also, the evidence of increasing global warming due to greenhouse emissions has raised concern for most developed and developing countries. All these concerns have increased the importance of research for fossil-free alternatives. Biomass is a renewable energy source that is widely available (Sarkar and Praveen 2017; Agrawal and Verma 2022). Due to environmental concerns and rising energy demands worldwide. Biomass utilization in mainstream energy usage attracts much attention. Biomass is composed of hemicellulose (20–35 wt.%), cellulose (30–50 wt.%), and lignin (15–35 wt.%) (Cheng et al. 2016; Kumar et al. 2020). Apart from carbon-based components, biomass also contains a small amount of nitrogen, sulfur, and inorganic composition ash. As a result, when compared to conventional fossil fuels, biofuel burning produces fewer toxic gas pollutants such as nitrogen oxides (NOx), sulfur dioxide (SO2), and soot. Furthermore, biomass fuel combustion can produce zero or negative carbon dioxide (CO2). emissions since CO2 generated from bio-oil combustion can be recycled back into the plant through photosynthesis (Debalina et al. 2017; Goswami et al. 2020a, 2021b). Different routes are available by which biomass can be converted into biofuel. These include thermal, biological, and physical processes. Among the different available processes, pyrolysis has emerged as a promising technology for producing liquid fuel products due to its storage, transport, and versatile applications such as combustion engines, boilers, and turbines. Managing solid biomass and waste is a

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complex and cost-intensive task that encourages pyrolysis research. However, it is still in its early stages of development and faces various technological and economic challenges to compete with existing fossil fuel-based technologies (Tang et al. 2020; Goswami et al. 2020b). An extensive investigation of the production of bio-liquids and other products using different biomass species has been carried out in the past. The quality bio-oil production from different biomass is challenging and requires the application of advanced technologies such as catalytic pyrolysis. Thermochemical conversion techniques can efficiently and economically transform biomass into energy-rich compounds used in various applications (Mandapati and Ghodke 2021). Different routes are available for converting biomass into usable fuel. These technologies include thermochemical conversion of biomass via combustion, pyrolysis, and gasification. The disadvantage of the gasification process is that the produced synthesis gas needs to be utilized at the place of production as it becomes economically inviable or costly for storage and transportation through pipelines (Mandapati and Ghodke 2020). The chapter discusses the different routes of pyrolysis and its technical specifications and the advances in the pyrolysis technology used for biomass conversion. The chapter covers a detailed description of the pyrolysis technique, its technical specifications, and the technology involved. Pyrolysis has emerged as an essential technology in turning biomass into solid, liquid, and gaseous fuels. A yield of around 60–65% bio-oil production has been reported in literature through the pyrolysis process utilizing a fluidized bed reactor.

7.2

Pyrolysis

Pyrolysis is the process in which both thermal and chemicals effects the conversion of organic materials in an oxygen-depleted atmosphere such as nitrogen. It is an endothermic reaction (Basile et al. 2015). The term pyrolysis was derived from two Greek words: pyro, which means fire, and lysis, which denotes degradation into essential components. During pyrolysis, components of biomass begin to decompose at 350  C–550  C due to rapid heating and progress to 700  C–800  C, resulting in the generation of a variety of products such as liquids, solids, and gases (Das et al. 2015). Biochar and bio-oil are the solid carbon-rich product and the volatile fraction of pyrolysis that is partly condensed to a liquid fraction product, respectively. The pyrolysis process produces hydrogen, carbon dioxide, methane, and carbon monoxide, among other gases (Moorthy Rajendran et al. 2020). These products are intriguing because they could be used as alternative energy sources. Pyrolysis has emerged as a critical method in converting biomass into solid, liquid, and gaseous fuels. The literature reported a yield of roughly 60–65 wt.% bio-oil generation through the fast pyrolysis process using a catalytic fluidized bed reactor. However, pyrolysis-produced bio-oil has numerous unfavorable qualities that directly use bio-oil as an engine fuel challenge. Thus, bio-oil is enhanced by the hydrotreating

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or catalytic cracking process to make it compatible as a drop-in fuel molecule (Shihadeh and Hochgreb 2000).

7.2.1

Different Routes of Pyrolysis

Pyrolysis processes may work efficiently under a variety of conditions. Thus, there are various pyrolysis processes, such as fast, slow, flash, intermediate, ultra-flash, vacuum, and catalytic pyrolysis (Wei et al. 2021). Especially, vacuum pyrolysis occurs at negative or low pressures, whereas another pyrolysis of biomass occurs at atmospheric or controlled positive pressure (Chintala et al. 2018). The ideal primary fuel for the pyrolysis process is waste material, such as forest debris, woody biomass, and agricultural waste. Longer residence times and high temperatures increase biomass conversion to gas. Pyrolysis conditions can be varied depending on the desired product type (Bridgwater 2012; Dhyani and Bhaskar 2017). Several different modes of biomass pyrolysis depending on conditions, such as heating rate and residence time of biomass, are being actively developed (Hu and Gholizadeh 2019; Kiliç et al. 2014; Adelawon et al. 2021; Dhyani and Bhaskar 2017). However, the most generally used classification of pyrolysis processes is slow, fast, and flash pyrolysis.

7.2.1.1

Slow Pyrolysis

Slow pyrolysis or conventional pyrolysis has been used to convert diverse feedstock biomasses into charcoal or biochar at slow heating rates for a lengthy residence time, around 5–30 min, and at temperatures below 300  C since the beginning of the pyrolysis process (Ahmad et al. 2014). Feedstocks, wheat straw, pinewood, dried algae, and green garbage were employed. This procedure can also make bio-oil or liquid fuels. Slow pyrolysis is typically carried out at atmospheric pressure. The heat required for the process is provided from an external source, usually produced from partial combustion or combustion of the produced gases or biomass feedstock (Ghodke and Mandapati 2019). The process results in the development of vapor phase components that continue to react, resulting in the formation of charcoal and other liquid products. Slow pyrolysis produces roughly 35 wt.% biochar, 30 wt.% bio-oil, and 35 wt.% gaseous products. However, due to technological restrictions such as cracking of the primary product due to high residence time (Demirbaş 2005; Jahirul et al. 2012), bio-oil is produced through slow pyrolysis is not suitable for direct use as a liquid fuel.

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Intermediate Pyrolysis

Intermediate pyrolysis is a type of pyrolysis that occurs halfway between fast and slow pyrolysis. It has an excellent product distribution and may be employed in the coproduction of biochar, bio-oil, and gas (Meng et al. 2019). Intermediate pyrolysis is flexible to diverse materials and has good product distribution. The product is a two-phase separable bio-oil, high quality, and biochar compared to other pyrolysis categories (Bridgwater 2003; Kumar et al. 2021). Researchers have also discovered that it has easily distinguishable liquid phases, with the organic phase exhibiting biodiesel-like qualities. The organic phase can be combined with up to 50 wt.% (Xiong et al. 2009). Its aqueous phase, which comprises C2–C6 sugars, hydroxy acids, oligomers, and water-soluble phenols, is also effective in manufacturing biogas and ethanol. The process has the advantage of requiring less bio-oil upgrading than quick pyrolysis oil and allowing for complete usage of all products. Because of the extensive interaction with steam, intermediate pyrolysis can treat high moisture content feedstock, and when this happens, the biochar takes on the properties of activated carbon (Gao et al. 2020).

7.2.1.3

Fast Pyrolysis

Fast pyrolysis is the rapid thermal degradation of biomass at more excellent heating rates, such as 1000  C min 1. Fast pyrolysis has a short vapor residence time of less than 2 s (Bhattacharya et al. 2009). Bio-oil is the main product of rapid pyrolysis. However, the amount of product created depends on the feedstock composition, ranging from 60% to 75% oily products, and 10–20 wt.% gaseous (CH4, CO, CO2, H2, and light hydrocarbons), and 15–30 wt.% solids products. Rapid heating and quenching produce bio-oil, and the high reaction rates reduce char formation and favor the generation of either gas or liquid products. Higher temperatures, heating rate, short vapor residence time, rapid cooling of vapors for high bio-oil production, and precise control of reaction temperature are the essential characteristics of the fast pyrolysis process (Collins and Ghodke 2018). Furthermore, a fast pyrolysis process requires reducing the water content of feedstock and reducing the particle size to less than 2 mm. In fast pyrolysis, rapid and systematic separation of solids (char), rapid gas removal, and cooling pyrolysis product favor the formation of bio-oil or liquid products (Ponnam et al. 2021). This liquid product can be readily and inexpensively transported and stored, decoupling solid biomass handling from consumption (Malode et al. 2021).

7.2.1.4

Flash Pyrolysis

Ultra-fast pyrolysis is another name for the process. Flash pyrolysis is a thermal breakdown of large molecules into smaller molecules that occurs in an inert

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Table 7.1 Summary of the classification of the pyrolysis method Method Slow pyrolysis Ultra-fast/flash pyrolysis Fast pyrolysis

Residence time 5–30 min

Temperature ( C) 400–500

Heating rate ( C/s) 10

500

0.5–2 s

500–650

100

Major bioproducts Char, bio-oil (tar), gases Bio-oil and gases Char, good bio-oil, gases

atmosphere such as nitrogen at a rapid heating rate of 1000–10,000  C per minute and residence time is less than 1 s. An excellent feed particle size (2 mm) is required to perform this procedure at an optimal rate. The degradation of biomass produces a significant amount of coke and aerosol vapors during flash pyrolysis. A dark brown liquid (bio-oil) is produced after cooling and condensation, with a heating value of half diesel. In contrast to prior procedures, this is a novel technology with wellregulated parameters that generate high liquid yields (Gandidi et al. 2018). Flash pyrolysis in fluidized bed reactors can produce a high liquid output of up to 75 wt.% bio-oil (Gómez et al. 2018). However, there are certain drawbacks to flash pyrolysis, such as low thermal stability, particles in the oil, and the oil's corrosiveness, which results in the generation of oils with high viscosity, density, low calorific value, and carbon residues (Huang et al. 2013). Table 7.1 depicts the summary of different pyrolysis technologies available and can be used to produce different bioproducts.

7.2.2

Technical Specifications: Pretreatment, Characterization, and Mechanism

7.2.2.1

Pretreatment

Pretreatment is an important stage in the biochemical and thermochemical conversion of biomass. It requires structural changes to overcome biomass's recalcitrance. Improved biomass features are necessary to increase the biomass's energy use efficiency (Wu et al. 2021; Bhardwaj and Verma 2021). The primary treatment process requires the heating of lignocellulosic biomass (cellulose, hemicellulose, and lignin) to convert into polymeric and aromatic constituents. In contrast, the heterogeneity in atoms and inorganic oxides element components of biomass act as catalysts, resulting in the generation of a biofuel product with different carbon structures and significant reforms that increase yield during the bioconversion process (Kim 2018; Kumar and Verma 2020a, b). Current pretreatment systems have two significant challenges: high costs and obtaining a processed product with essential component degradation. Past and ongoing research and development efforts have failed to meet these problems. Pretreatment treatments must be tailored to the type of biomass and how it will be utilized in bioconversion and biorefinery

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Fig. 7.1 Different pretreatments methods for biomass

processes. Figure 7.1 depicts the types and pretreatment processes required for biomass conversion technologies (physical, chemical, and biological). The subsections that follow go into the various types of preprocessing in greater depth.

Physical Treatment Physical pretreatment of biomass aims to improve the surface area and pore size by reducing particle size through mechanical comminution. Physical processing reduces cellulose crystallinity and polymerization degree in lignocellulosic materials. Prior to both enzymatic and thermochemical biofuel production, this step is required. However, little is known about how physical preprocessing procedures function, particularly how biomass chemical content or bond structure are affected. The lignocellulosic biomass application determines the physical pretreatment technology to be undergone or utilized. Biochemical conversion of lignocellulosic biomass, for example, should undergo a reduction in particle size through milling, shredders, and cutters in order to improve biochemical digestibility. Milling of biomass is necessary before use in the thermochemical conversion process to reduce the size of biomass. Palletization, densification, and heat treatment torrefaction are the prior pretreatment process of thermochemical conversion technologies. Prior size reduction is essential in both biochemical and thermochemical conversion pathways to minimize mass and heat transfer constraints. Another physical pretreatment approach is chipping when employing waste wood biomass or agricultural residue as a feedstock in thermochemical conversion technologies. The thermochemical conversion technologies require feedstock with 50 mm or less diameter, feedstock with 50 mm or less is required. Torrefaction, densification, and pelletization are examples of physical pretreatment procedures for biomass (Reckamp et al. 2014; Chi et al. 2021). These preprocessing procedures use heat to cause changes in the biomass, resulting in improved biomass characteristics. The physical pretreatment procedure has the

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drawback of removing the lignin content from lignocellulosic biomass materials, rendering the cellulose composition inaccessible. Another disadvantage is the high heat-energy requirements for pretreatment and not economically feasible commercial use. According to research findings, the process of removing lignin from lignocellulosic biomass materials can be led to an increase in the energy demand. Thus, thermal pretreatment treatment method could influence the total energy effectiveness/cost of a biorefinery process (Lewandowski et al. 2020).

Chemical Pretreatment In chemical pretreatment methods, organic or inorganic chemical compounds are used to disrupt the chemical bonds of lignocellulosic biomass by interacting with the intrapolymer and interpolymer bond connections of the organic components are known as chemical pretreatment of biomass. Biomass, particularly lignocellulosic components, is considered recalcitrant because it is resistant to chemical breakdown. The structural diversity and complexity of biomass such as its crystalline nature and the degree of lignification, all contribute to its recalcitrance (Mai et al. 2014; Bhardwaj et al. 2020). In preliminary chemical pretreatment method, the chemical structural recalcitrance of the lignocellulosic nature of biomass is disrupted, resulting in cellulose crystalline phase reduction along with lignin breakdown. Before biochemical conversion, chemical preprocessing of biomass, particularly lignocellulosic biomass, is frequently employed to extract the biopolymeric constituents of the feedstock. Acids, alkalis, organic solvents, and ionic liquids are some of the chemicals that have been used to pretreat biomass chemically and have had a significant impact on its structure (Basak et al. 2020).

Biological Pretreatment Biological preprocessing/pretreatment of the organic composition of biomass is always related to the activity of synthetic enzymes or enzymes produced from an organism that potentially can break down or depolymerize the hemicellulose, cellulose, and lignin components of biomass. The biological pretreatment method has many benefits over other physical/chemical pretreatment processes are particularly uses energy consumption and produces little or no toxic chemical. Biological pretreatment produces a high yield of required/valuable products. But major disadvantages are substrate and process reactions are very sensitive (Cao et al. 2013; Agrawal and Verma 2020). However, its significant drawbacks in biological pretreatment are that the method chosen was too slow and requires meticulous management of fungus growth conditions and the enormous amount of area required to complete the process (Kan et al. 2016). It was observed from the literature that the residence period or time required for biological activities in pretreatment was between 11 and 15 days. Moreover, because microorganisms consume the organic components of biomass, biological pretreatment operations have technological

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challenges and are seen as less economically appealing than other pretreatments (Robak and Balcerek 2018; Chintala et al. 2019). In the biological pretreatment of biomass, different fungi such as white-rot, brown-rot, and soft-rot fungi are used widely. Apart from fungi, actinomycetes and bacteria are used to pretreat the biomass. The fungi are notably applied to eliminate hemicellulose and lignin composition of biomass while causing minor damage to cellulose biomass. White-rot and brown-rot fungi have a wide degradation mechanism for gaining access and destroying the lignocellulosic biomass such as waste wood and agricultural residue. Their extraordinarily powerful metabolism or mechanism has been successfully employed in industrial commercial operations. White-rot and brown-rot fungi have been demonstrated to brighten hardwood kraft pulp, potentially lowering bleaching chemical costs and reducing the environmental effect of paper manufacturing operations (Phillips et al. 2017; Ghodke and Mandapati 2017; Bhardwaj et al. 2021). Even though various preprocessing methods have been studied and more are in progress, evaluating pretreatment technologies is challenging. The primary reasons for challenging pretreatment techniques are upstream and downstream processing of biomass. Additionally, capital expenditure, waste treatment systems, and complicated chemical recycling (Shen 2015; Ghodke 2021). The thermochemical conversion uses a variety of feedstocks, including wood waste, energy crops such as sweet sorghum, short rotation forestry, and agricultural residues. The critical technical parameters for thermochemical processing suitability are ash, moisture, and other physical characteristics. The two most essential economic elements are cost, including collecting, production, transportation, and availability. Competing uses such as pulp and combustion, board manufacture, recycling, and material recovery, rather than energy recovery, are also a concern.

7.2.3

Feedstock Drying

In most circumstances, pyrolysis necessitates a feedstock with a moisture level of less than 15 wt.% although there is a trade-off between moisture levels and conversion production efficiency. The moisture content essential for conversion differentiates between conversion plants. When biomass is delivered, the moisture level will be 50–60 wt.% range (Praveen et al. 2015). Passive drying during summer storage can cut this by 30%. The moisture content of a silo can be reduced to as low as 12 wt.% with active silo drying. Drying can be done in various ways, including near-ambient solar drying, waste heat flows, and specially constructed dryers that operate at the location. Commercial dryers come in various shapes and sizes, but rotary kilns and shallow fluidized bed dryers are the most prevalent.

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Characteristics of Feedstocks

Table 7.2 summarizes the essential physical features of biomass. Notably, the relatively low bulk density, high moisture content, and wide particle size range are notable traits.

7.2.3.2

Biomass Characterization

Biomass heterogeneity is still a natural feature. The biomass quality determines the possibility and viability of extracting products from it. The two conversion techniques such as biochemical and thermochemical conversions are primarily applied to recover commercially valuable products from biomass. The attributes of biomass also affect the conversion route chosen. Thus, characterization is required to effectively understand the underlying physicochemical properties of biomass. Depending on its characteristics, one can evaluate its suitability for bioconversion. The physical properties are critical for the efficient use of biomass in biofuel production methods (Dutta et al. 2015). However, the fundamental organic components of biomass influence its features, which vary based on the source of biomass composition, biomass source, species, soil condition, climatic condition, and other factors. Depending on the end-use of biomass, proximate and final analyses are routinely determined and reported using a variety of analytical processes. The data is crucial for assessing biomass's diverse application potential, notably its energy production potential when third-generation biomass is used as a biofuel in thermochemical conversion operations ( Sharma et al. 2020). Among the characteristics analysis techniques used in physicochemical characterization investigations involving organic composition are the following: • • • • • • • •

XRD- X-ray diffraction analyzer FTIR- Fourier transforms infrared spectroscopic analyzer TGA- Thermogravimetric analysis analyzer NMR- Nuclear magnetic resonance spectroscopy CHNS-O analyzer DSC-Differential scanning calorimetry SEM-Scanning electron microscope Py-GC/MS- Pyrolysis-gas chromatography/mass spectrometry

Table 7.2 Essential physical features of biomass Feedstocks Moisture content (%) Density (kg/m3)

Forestry residues 20–50 350

Poplar tree 10–40 450

Waste wood 5–30 350

MSW 20–30 450

Straw 5–25 250

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Future Prospects

The replacement of fossil fuels with biomass resources on a wide scale is a hot topic not just for energy generation but also for manufacturing chemicals, bioproducts, and materials. Moreover, and since biomass is extensively accessible, commercial products/biofuels from lignocellulosic materials can be produced in most geographic locations. Biomass characteristics determine the economic viability and effectiveness of the value-added product production, the primary treatment method used. However, biomass heterogeneity, high capital and operating costs associated with bioconversion, and the mechanisms underlying the biomass conversion process are all issues with using biomass for heat, chemical products, and fuels generation. As a result, efforts should be made at all levels to build more user-friendly and costeffective technologies to stimulate the broad use of biomass and attract investment in this field. Furthermore, because ideal biomass pretreatment conditions are rarely published, nothing is known about them.

7.4

Summary

Biomass preprocessing and characterization is critical in determining that biomass materials are utilized effectively in biofuel production. An improved comprehension is required of the origins of biomass and its recalcitrance. Further the impact of primary treatment to maximize the different biofuel production pathways. It requires an assessment of biomass using cutting-edge analytical techniques capable of providing knowledge. Based on the features of the pyrolysis products, criteria for assessing the feasibility of biomass for a pyrolysis conversion process to produce solid, liquid, and gaseous fuels are developed. The applicability of various biomass for pyrolytic transformation is investigated using this technique. It is discovered that various biomass is suitable for diverse applications such as combustion of fuel, liquefaction, gasification, and the production of char adsorbents depending on the chemical composition of biomass. Conflict of Interest All the authors declare that they have no competing interests.

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Chapter 8

Biomass Pyrolysis and its Multiple Applications Shivangi Pathak, Anil Kumar Sakhiya, and Priyanka Kaushal

Abstract In the twenty-first century, the growing demands of a modernized and growing population have led to the rapid expansion of agricultural and industrial sectors around the world. This expansion leads to produce a massive amount of biomass materials and various organic and inorganic pollutants. Biochar has promising potential in tackling such global concerns and can serve as a low-cost adsorbent for accomplishing sustainable development goals (SDGs). Biochar, a carbon-rich solid product, can be obtained by slow pyrolysis of biomass under an oxygen-limited atmosphere. Produced biochar can be used as an adsorbent to remove organic and in-organic pollutants from groundwater and industrial wastewater. However, biochar has a lower surface area and limited surface functional groups, which results in lower adsorption capacity. Hence, activation is required. Apart from adsorbent biochar is also used as a soil additive to sequestrate carbon to mitigate climate change and enhance its fertility and water retention capability hence lowering the frequency of irrigation in the field. In this chapter, the fundamentals of slow pyrolysis, its process parameters, product yield distribution, and various application of biochar will be discussed in detail. Keywords Pyrolysis · Biochar · Activation · Adsorbent · Soil amendment

Abbreviations ACB BSA CEC DCFC DFT

Araucaria columnaris bark Biochar soil amendment Cation exchange capacity Direct carbon fuel cell Density functional theory

S. Pathak · A. K. Sakhiya · P. Kaushal (*) Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_8

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dw fw GRSB JPB MB MBC OM PAHs PBB PRSB RE RO SA SPS SSS TC TPH WHC

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dry weight basis Fresh weight basis Granular Rice Straw biochar Jujube pit biochar Methylene blue Magnetically modified rice husk biochar Organic matter Polycyclic aromatic hydrocarbons Powdered bamboo biochar Powdered Rice Straw biochar Removal efficiency Reverse osmosis Sulfonamides Spent P. ostreatus substrate Spent shiitake substrate Tetracycline Total petroleum hydrocarbons Water holding capacity

Introduction

The world’s food, water, and energy demand are increasing rapidly as the global population increases. Large amounts of biomass waste (crop residues, forest residues, organic) are generated daily due to high living standards and economic development. Globally, the approximately 140 Gigaton biomass waste generated and managing such a huge amount of waste is a great challenge. Most of the biomass waste is discarded and open burning in the field, which negatively impacts the environment (Tripathi et al. 2019; Goswami et al. 2020a; Agrawal and Verma 2022). The conversion of biomass waste to biochar through pyrolysis is a feasible solution and creates value addition in society. Biochar is a “carbon-rich solid product” formed by pyrolyzing biomass at high temperatures (300–600
C) in an inert environment (Sakhiya et al. 2020; Goswami et al. 2020b, 2021). Biochar can be used for a variety of energy and environmental purposes, including clean solid fuel, fuel cells, catalysts, soil amendments, composting additives, groundwater and wastewater purification, air purification, carbon sequestration, hydrogen storage, etc. (Dillon and Heben 2001; Sakhiya et al. 2021a, b; Baghel et al. 2022). The pyrolysis product yield distribution is influenced by a variety of factors, including pyrolysis temperature, heating rate, residence time, and particle size. Presently, biochar is used as an economical adsorbent for removing organic (PAHs, pesticides, dye, surfactants, pharmaceutical) and inorganic (heavy metals such as As, Ni, Pb, Zn, and Cu) contaminants from groundwater and wastewater (Cha et al. 2016). Moreover, biochar has a good water

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holding capacity and nutrient retention capacity, improving agriculture sustainability and product yield. Biochar produced from agricultural waste and used in the soil can return the nutrients and develop a circular economy (Zhou et al. 2021). However, there are certain limitations to using biochar as an adsorbent and other environmental application due to cation exchange capacity, limited surface functional groups, low porosity, and surface area. The activation is a process that improves the physicochemical properties of biochar using different activation agents and heating in the temperature range of 500–900
C. According to the report “Global Activated Carbon Market Forecast & Opportunities 2017,” demand for activated carbon is predicted to grow at a rate of higher than 10% per year for the next halfdecade, reaching a market value of 3 billion dollars by 2025 (Park et al. 2013). The sustainable utilization of biomass to produce bioenergy and activated charcoal would not only assist in alleviating the environmental issues due to coal mining but also cuts down the price of producing efficient sorbents. This chapter describes the biochar production through the pyrolysis, types of pyrolysis process, parameters influencing the pyrolysis. Activation of biochar via physical and chemical activation is also discussed in detail. Additionally, biochar has a wide array of uses such as solid fuel, catalyst, the adsorbent in water purification, additives in composting, and hydrogen storage were described in detail.

8.2

Pyrolysis

Pyrolysis is a thermochemical conversion technology in which biomass is thermally heated at elevated temperature and produced three different products: biochar, bio-oil, and gas. In addition to its efficacy, pyrolysis produces multi-products compared to other thermochemical conversion processes (Tripathi et al. 2016).

8.2.1

Physics of Pyrolysis

In the pyrolysis process, organic material is thermally decomposed under an oxygenlimited environment. It’s a particularly complicated process with a lot of various reactions in the reacting zone. Biomass contains three major constitutes, including hemicellulose, cellulose, and lignin (Kumar and Verma 2021a, b). The heating of biomass, biomolecules of hemicellulose, and cellulose break down in the form of volatile matter and produce bio-oil by condensation. The heating of biomass under an oxygen-limited environment allows a rise in temperature greater than its limit thermal stability and produces a more stable solid product called biochar. The pyrolysis process mainly consists of two steps, i.e., primary and secondary pyrolysis. During primary pyrolysis, heat causes biomass molecules to cleave and devolatilize, forming numerous functional groups such as hydroxyl, carboxyl, and carbonyl. The process of devolatilization takes place by dehydration of biomass,

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followed by decarboxylation and dehydrogenation. Secondary pyrolysis begins after the completion of the primary pyrolysis process in which heavy hydrocarbon breakdown into condensable (bio-oil) and non-condensable gases (CO, CO2, H2, and CH4). The cracking of heavy hydrocarbon compounds can be presented by the following reaction (Tripathi et al. 2016): ðC6 H6 O6 Þn ! ðH2 þ CO þ CH4 þ . . . þ C5 H12 Þ þ ðH2 O þ CH3 OH þ CH3 COOH þ . . .Þ þ C

ð8:1Þ

Where the initial part of the products in the reaction represents the non-condensable gases, the later part represents the mixture of condensable gases, and the last part is the char yield.

8.2.2

Types of the Pyrolysis Process

The process parameters help in classifying the pyrolysis process into four categories: slow, fast, flash, and intermediate. Each type has its own set of benefits and drawbacks. The basic characteristics of each type of pyrolysis and the operating parameters are discussed in the following section. Table 8.1 represents the types of pyrolysis processes based on operating conditions.

8.2.2.1

Slow Pyrolysis

It is a conventional method and widely used to produce charcoal in history and characterized by a modest heating rate and a prolonged residence time. It is generally carried out from 400–600
C to get higher biochar yield with a small amount of Table 8.1 Operating conditions of types of pyrolysis processes Operation condition Temperature (
C) Heating rate (oC/min) Residence time (min) Particle size (mm) Reference

Slow 550–950

Fast 850–1200

Flash 900–1300

Intermediate 400–650

0.1–1.0

10–200

>1000

1.0–10

300–500

0.5–10

800
C) due to the higher activation energy requirement for cracking. Catalyst takes a crucial position in biomass gasification by increasing the quality and yield of gaseous biofuel at the expense of undesirable tar and char, at a lower process temperature at a faster rate. Among the different catalysts, commercial Ni-dolomite combinations exhibit remarkable tar reforming capability of 89–99% by mutually mitigating the deactivation of nickel and CH4 reformation insensitivity of dolomite. In this view, Ni-dolomite combinations can economically nourish producer gas through simultaneous tar reformation and CO2 sorption. Recent practices and future scopes for catalytic performance enhancement through new catalyst configurations and novel technologies are also discussed. Keywords Biomass gasification · Gasifier · Tar reforming · Catalysts · Deactivation

Abbreviations ASH ATR daf ER FC

Ash Autothermal reforming Dry ash free Equivalence ratio Fixed carbon

R. S. (*) Department of Mechanical Engineering, PES College of Engineering, Mandya, Karnataka, India Deepanraj Department of Mechanical Engineering, Jyothi Engineering College, Thrissur, Kerala, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_9

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FT GR GT M ms MTG Nm3 PAH SBR VM WGS

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Fischer–Tropsch Gasifying ratio Gas turbine Moisture Milliseconds Methanol-to-gasoline Normal cubic meter Polycyclic aromatic Steam to biomass ratio Volatile matter Water gas shift

Introduction

Environmental sustainability and fossil fuel utilization are two inevitably conflicting requirements for the better being of mankind. Complete reliance on conventional energy sources, to meet the global energy demand, leads to both an energy crunch and adverse environmental issues due to the surplus emission of greenhouse gases like carbon dioxide (Goswami et al. 2020, Goswami et al. 2022). The estimated rise in world energy demand for 2021 is 4.6%, with an associated increase of 33 billion tonnes of carbon dioxide emission. Thus, depleting nature of conventional energy sources and their unfavorable environmental impacts encouraged energy extraction from replenishable energy sources. Consequently, in 2020, the global utilization and electricity generation from renewable resources is raised by 2% and 3%, respectively. Moreover, an 8% improvement in renewable energy to electricity conversion is targeted in 2021 to attain an output of 8300 TWh (Rupesh et al. 2021). Among the renewable energy sources, biomass is the highest energy distributer, approximating 15% of primary global energy consumption. However, the practical applicability of biomass can be ensured by tackling its low energy density through sustainable and eco-friendly energy conversion routes. Among the feasible bio-chemical, physiochemical, and thermochemical conversion methods, thermochemical means (pyrolysis, combustion, and gasification) are attractive because of their faster and continuous conversion. In thermochemical conversion, biomass gasification is a potential and practically feasible pathway to extract and concentrate energy embedded in biomass. In gasification, biomass is transmuted to gaseous fuels and upgraded chemical products in sub-stoichiometric conditions, without leaving any net adverse impact on the environment. The gaseous fuel obtained from gasification is mainly a mixture of hydrogen and carbon monoxide, turned into synthesis gas or syngas, which can also serve as a chemical feedstock for synthesizing other liquid fuels and value-added chemicals. In biomass gasification, the extent of biomass conversion and, the quality and quantity of the fuel generated are significantly influenced by the reactor, operating parameters, and presence of catalysts. This chapter deals with

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various aspects of biomass gasification and the role of catalysts in augmenting the quality and yield of syngas.

9.2

Biomass Gasification

Biomass is non-fossilized, renewable, and biodegradable organic matter derived from animals, plants, or microorganisms. Major biomass sources consist of terrestrial and aquatic biomasses, agricultural, municipal, and solid waste, and forest residue (Sikarwar and Zhao 2017; Kumar and Verma 2021a, b). Especially, non-edible fibrous plant derivatives composed of lignin, cellulose, and hemicellulose, known as lingo-cellulosic biomass, threaten proper waste management or lead to adverse environmental impacts on direct combustion. This can be properly addressed by converting them to energy through appropriate energy conversion routes (Bridgwater 2003; Bhardwaj et al. 2021a, b; Bhardwaj and Verma 2021). Biomass gasification is the sub-stoichiometric thermochemical transmutation of biomass to gaseous fuel and upgraded chemical products. During gasification, biomass is subjected to thermal decomposition followed by chemical reactions with the participation of a medium, known as a gasifying agent, in a reactor termed as gasifier. Basically, gasification is a four-stage process comprising drying, pyrolysis, partial oxidation, and reduction. The steps in gasification are mentioned in Table 9.1. The gasification product’s yield firmly relies on the biomass, gasifying medium, reactor configuration, and process parameters. In general, the gasification products comprise a non-condensable gaseous mixture mainly consisting of H2, CO, CO2, CH4, known as producer gas, condensable higher hydrocarbon species collectively termed as tar, and solid constituents such as char and ash. The overall biomass gasification process can be represented as:

Table 9.1 Different steps in biomass gasification (Sansaniwal et al. 2017) Name Drying

Temperature range (
C) 100–150

Pyrolysis

150–700

Partial oxidation Reduction

700–1500 800–1100

Process The moisture content in biomass (5–35 wt.%) is transformed into steam Thermal dissociation of biomass structural constituents such as lignin, cellulose, and hemicellulose into condensable and non-condensable volatiles and char Sub-stoichiometric combustion or oxidation of pyrolysis products that release heat to drive the other three endothermic steps The various condensable and non-condensable gases, char, and gasifying agents chemically interact to yield gaseous constituents of producer gas

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Biomass þ Gasifying agent þ Heat ! H2 þ CO þ CO2 þ CH4 þ H2 O þ Tar þ Char

9.2.1

ð9:1Þ

Gasification Chemistry

The biomass gasification chemistry is too intricate with numerous reactions and is not yet completely explored (Pfeifer and Hofbauer 2008). The main homogenous and heterogeneous chemical reactions in gasification are listed in Table 9.2. The Table 9.2 Main gasification reactions (Basu 2010; Valderrama Rios et al. 2018;Silva et al. 2019)

S. No. Reaction name 1 Drying 2 Pyrolysis Oxidation reactions 3 Char oxidation 4 Partial char oxidation 5 Hydrogen oxidation 6 Carbon monoxide oxidation 7 Methane oxidation 8 Tar partial combustion Reduction reactions 9 Boudouard 10 Hydrogasification 11 Char steam reforming (Water gas reaction) 12 Water gas shift (WGS) reaction 14 Methanation reactions 15 16 Tar decomposition reactions 17 Thermal cracking 18 19

Steam reforming

20 21 22

Dry reforming Hydrogenation Carbon formation

Stoichiometry Wet Biomass + Heat ! Dry Biomass + H2O Dry Biomass + Heat ! Gas + Tar + Char

Enthalpy of reaction (kJ/mol) – –

C + O2 ! CO2 C + 0.5O2 ! CO H2 + 0.5O2 ! H2 CO + 0.5O2 ! CO2

394 111 242 283

CH4 + 2O2 ! CO2 + 2H2O CmHn + (0.5m + 0.25n) O2 ! mCO + 0.5nH2O

520 –

C + CO2 $ 2CO C + 2H2 $ CH4 C + H2O $ CO + H2

+172 74.8 +131

CO + H2O $ CO2 + H2

41.2

2CO + 2H2 ! CH4 + CO2 CO + 3H2 $ CH4 + H2O CO2 + 4H2 ! CH4 + 2H2O

247 206 165

CmHn ! aCm  xHn  y + bH2 CmHn ! 0.25Cm  xHn  y + 0.25(4n  m) H2O
 Cn Hm þ nH2 O ! n þ m2 H2 þ nCO Cn Hm þ nCO2 ! m2 H2 þ 2nCO CnHm + 2(n  0.25m)H2 ! nCH4 CnHm ! nC + 0.25mH2

– – – – – –

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reactions can be classified as drying, devolatilization or pyrolysis, oxidation, reduction, and tar reforming reactions. The extent of involvement of each reaction in the process relies on biomass composition, type of gasifying medium, reactor design, and process parameters. Overall, gasification is an endothermic process; is termed autothermal if the heat required is supplied by the combustion reactions, within the reactor, or allothermal if supplied externally.

9.3

Gasifying Agents

The role of gasifying medium or agent is pivotal in the yield and quality of gasification products. Air, steam, air-steam (both air and steam), oxygen, and steam-oxygen (both steam and oxygen) are the mainly used gasifying agents in biomass gasification. A comparison of the merits and limitations of air, steam, and oxygen as gasifying agents is listed in Table 9.3. In general, the quantity of gasifying agent used is referred to using non-dimensional terms defined with respect to the quantity of biomass used. Equivalence ratio (ER), steam to biomass ratio (SBR), and gasifying ratio (GR) are the non-dimensional terms, respectively, used for air, steam, and steam-oxygen mixture. Actual air  fuel ratio Stoichiometric air  fuel ratio Mass of steam Steam to biomass ratio ðSBRÞ ¼ Mass of biomass

Equivalence ratio ðERÞ ¼

Gasifying ratio ðGRÞ ¼

Mass of ðsteam þ oxygenÞ Mass of biomass

ð9:2Þ ð9:3Þ ð9:4Þ

Table 9.3 Comparison of different gasifying agents (Sansaniwal et al. 2017) Agent Air

Oxygen

Steam

Merits • No external heat is required as heat for gasification is obtained from partial combustion. • Economical • The gasifying medium can enable partial oxidation, so no external heat is required. • H2, CO, and CH4 enriched producer gas with minimum tar and no N2 dilution • Better carbon conversion • Producer gas enriched with H2 • A higher H/C ratio of producer gas is suitable for the synthesis of liquid fuels

Demerits • Low calorific value due to N2 dilution • Can be used only for direct burning applications • Comparatively, expensive handling is required • Safety issues • •

Higher tar content Energy-intensive as external heat is required to enable gasification

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Table 9.4 Influence of gasifying medium on gas composition Medium Air Steam Steam and oxygen

Gas composition (vol.%) CO CO2 H2 5–16.3 9.9–22.4 9.0–19.4 38–56 17–32 13–17 13.8–31.7 42.5–52 14.4–36.3

CH4 2.2–6.2 7–12 6–7.5

N2 42–62 0 0

Gas yield (Nm3/kg daf) 3.7–61.9 60–95 2.2–46

Air gasification is autothermal as the heat input for gasification is derived from partial oxidation of biomass. But the quality of the producer gas obtained is low as it is diluted with N2 contained in the air. Consequently, the heating value of the producer gas from air gasification is limited to 4–7 MJ/Nm3. Self-sustained gasification with H2, CO, and CH4 (without N2) enriched producer gas enables the achievement of a superior calorific value of 12–28 MJ/Nm3 in oxygen gasification. On the other hand, the use of steam results in producer gas with a higher H/C fraction having a calorific value (lower heating value) of 10–18 MJ/N m3 (Basu 2010). Combinations of different gasifying agents are used to merge their advantages to improve the producer gas characteristics. Gas yield and composition using different gasifying agents are depicted in Table 9.4.

9.4

Types of Gasifiers

The gasifier, the reactor in which gasification takes place, has a vital impact on the quantity and quality of producer gas generated. The design, construction, and operation of gasifiers differ depending on the biomass attributes (moisture content, ash content, size, shape, etc.), quality, and yield of syngas to suit various downstream applications. The reactor is generally classified into fixed bed, fluidized bed, entrained flow, and plasma gasifiers.

9.4.1

Fixed Bed Gasifier

9.4.1.1

Updraft Gasifier

In an updraft gasifier, the gasifying agent supplied from the bottom reacts with the descending biomass loaded from the top to generate producer gas rising through the bed. The reactor is termed updraft, due to the upward flow of gas, whereas the opposing gas flow relative to biomass in the reactor signifies its alternate name countercurrent gasifier (Loha et al. 2018). In updraft gasifiers, biomass is sequentially subjected to drying, pyrolysis, reduction, and partial oxidation in the respective

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Fig. 9.1 Updraft gasifier

distinct zones to generate producer gas as shown in Fig. 9.1 (Ren et al. 2019). Generally, in the oxidation zone, the ash is melted at 1200
C. The flow of raw gas generated through a relatively low-temperature pyrolysis zone laden the producer gas with uncracked tar precursors, resulting in lower syngas yield with higher tar content. During this process, after passing through the fuel bed, the hot gases leave at a lower temperature range of 200–400
C (Basu 2010; Surisetty et al. 2012). As the gas flows from a high-temperature oxidation zone to low-temperature pyrolysis and drying zones, the heat content in it can be utilized for drying and pyrolysis, thereby achieving improved thermal efficiency. Thus, updraft reactors can accommodate feedstock with higher moisture content (up to 60% by weight) and are well suited for downstream applications that can accommodate producer gas with high tar content, low dust at high flame temperature (Sansaniwal et al. 2017).

9.4.1.2

Downdraft Gasifier

In this gasifier, both biomass loaded from the top and gas generated move parallelly downward and hence the name downdraft gasifier. Due to this unidirectional movement of biomass and producer gas, this reactor is also known as a co-current gasifier (Sansaniwal et al. 2017). The tar concentration in the producer gas is relatively low due to the gas flowing from the low-temperature drying zone to the high-temperature

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Fig. 9.2 Downdraft gasifier

combustion zone and thereby cracking the tar formed from the pyrolysis more effectively (Bermudez and Fidalgo 2016). Thus, producer gas passing through the high-temperature (900–1000
C) zones aids in the partial combustion of tar and leaves the gasifier at 700
C (Basu 2010; Devi et al. 2020). However, pyrolytic gas combustion to support endothermic reactions lowers the heating values of produced gas (Ruiz et al. 2013). Moreover, incompetence in accommodating biomass with high moisture ( Ni/CeO2 > Ni/ZrO2 (Ren et al. 2020). Further improvement in tar reforming can be attained by combining support material with another metal. A higher benzene conversion is attained at 850
C, using bimetallic catalyst Ni/ZrO2 with La, Sr, and Fe which in turn improves tar conversion. (Keller et al. 2016). Furthermore, a bimetallic MgAl2O4-supported IrNi catalyst exhibits superior stability during steam reforming of methane and tar due to a substantial reduction in coking and sintering (Dagle et al. 2016).

9.8.4

Zeolites

Zeolite, a crystalline aluminosilicate material occupying ions and water molecules within the cavities in its structural framework, comes under the category of fluid catalytic cracking catalysts, which have the potential to eliminate tar. Zeolites are acidic in nature with better hydro-thermal stability, good nitrogen, and sulfur resistance, low cost, and easy regeneration ability (Buchireddy et al. 2010). Zeolites usually find their application as supports to Ni-based or other metal-based catalysts in tar reforming. Among the different variants of zeolites like Zeolite Y, Zβ, ZSM5, ZY-5.2, the variants ZY-5.2 and Zeolite Y show better naphthalene conversion due

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to their acidic nature and larger pore size, respectively. But, nickel-supported zeolite (Ni/ZY-80) can attain a naphthalene conversion rate of 99% with prolonged activity (Kathiraser et al. 2016). Moreover, the addition of Fe or Ni in mesoporous SBA-15 catalyst improves the formation rates of H2, CO, CO2, and CH4 whereas the addition of Ni in mesoporous MCM-41 can achieve the lowest coke deposit of 7 wt% of the catalyst, thereby, mitigating the major limitation of zeolite (Wu and Williams 2011a).

9.8.5

Activated Char Catalysts

Bio-char, a gasification residue concentrated with metals like K and Ca contributed from the base biomass feedstock, can act as a catalyst. These alkali or alkaline earth metals can promote WGS reaction and enhance tar reforming through H2O absorption (Hu et al. 2015). Moreover, tar reforming contributed by some functional groups containing oxygen in the bio-char makes it a suitable catalyst for biomass gasification (Shen et al. 2015). Furthermore, the tar removal efficacy of bio-char can be improved up to 97% by incorporating it with Ni (Wang 2013). A better gas yield of nearly 90 mg per g of biomass and carbon balance of 98% is attained using Ni dispersed brown coal char at a lower temperature range of 400–550
C (Li et al. 2010). Additionally, a more potent way of acquiring high-quality syngas is by combining char with Ni-Co catalyst, Ni-Co altering the quality and rate of syngas. (Yang et al. 2019).

9.9

Comparison of Different Catalysts

This segment deals with the comparison of different catalysts discussed in Sect. 9.8.

9.9.1

General Comparison

Initially, a general comparison of natural mineral, alkali metal-based, metal-based, zeolite, and char catalysts are incorporated, as in Table 9.7.

9.9.2

Comparison of Gas Yield and Tar Conversion

The tar reforming capability of a catalyst improves the yield and quality of syngas. The following section depicts the comparison of gas yield and tar conversion from different catalysts.

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Table 9.7 General comparison of various catalysts Classification Natural minerals

Catalyst Dolomite

Merits • Cheap, • Higher tar conversion rate

Olivine

• • •

Limonite

• •

Alkali metalbased

Alkali metal

• •

Metal-based

Ni-based

• • • •

Metal and Metal oxide

• • •

Zeolite

Zeolite

• •

Char

Bio-char

• • •

Mechanical strength Non-toxic Attrition resistance High tar reforming Inhibit coke formation Enhances water gas shift reaction Higher yield of H2 rich gas Higher tar reforming Ability to reform methane Enhances water gas shift reaction Can be used as both primary and secondary catalyst Ability to reform methane Higher benzene conversion Suppress coking and sintering Better hydrothermal stability Easy regeneration Ample availability Obtained as a by-product of gasification Promotes water gas shift reaction

Demerits • No methane reformation • Fragmentation, attrition, and elutriation • Limited tar reformation • Insensitive to methane reforming • Presence of impurities

References (Gil et al. 1997; Zhou et al. 2009; Ren et al. 2019)

•

Particle agglomeration Higher ash content Coking, sulfur poisoning, sintering

(Dayton 2002; Long et al. 2012)

• •

Expensive Deactivation due to sulfur poisoning

(Nishikawa et al. 2008; Ren et al. 2019)

•

Intricacy in synthesis

(Buchireddy et al. 2010; Ren et al. 2019)

•

Susceptible to gasification

(Li et al. 2010; Yao et al. 2016)

• •

(Rapagnâ 2000; Devi et al. 2005a; Pfeifer and Hofbauer 2008) (Li et al. 2007, 2010)

(Baker and Mudge 1984; Guan et al. 2016)

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Table 9.8 Comparison of natural mineral-based catalysts on gas yield Catalyst Pt -Ni/dolomite Fe-Ni/dolomite Co-Ni/dolomite Ni/dolomite Natural dolomite Calcined dolomite Natural olivine Calcined olivine

Reaction temperature (
C) 800 800 800 770 800 800 800 800

Gas yield (%) 79.19 78.12 49.53 69.1 44.67 61.33 36.60 46.67

References Chaiprasert and Vitidsant (2009)

Sato and Fujimoto (2007) Islam (2020)

Fig. 9.10 Comparison of noble metal-based catalysts on tar conversion (Tomishige et al. 2004)

9.9.2.1

Natural Mineral-Based Catalysts

The comparison of natural mineral catalysts on gas yield is given in Table 9.8. It is clear that the tar reforming capability of both natural dolomite and natural olivine is improved when calcinated. Consequently, the gas yield is increased by 37.2% and 27.5%, respectively, for calcined dolomite and calcined olivine when compared with their natural form. Thus, the acceptance of calcined dolomite interns of gas yield is 31.4% more than that of calcined olivine. Moreover, the tar reforming as well as gas yield of dolomite can be augmented by combining it with metals like Ni, Fe, Pt, Co, etc. The addition of Fe and Pt with Ni/dolomite contributes comparable improvement in gas yield, with the former prompting coking and the latter being expensive.

9.9.2.2

Metal-Based Catalysts

The comparison of noble metal-based catalysts on tar conversion is represented in Fig. 9.10 CeO2 has a vital role in enhancing tar conversion in a noble metal-based catalyst. Addition of CeO2 in Rh/SiO2 increased tar conversion by 21.12%. The tar

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Table 9.9 Comparison of nickel-based catalysts on gas yield Catalyst Ni/MgO NiO-MgO NiO/MgO Ni-Mg-Al Ni-Mg-Al-CaO Ni-Mg-Al-Quartz Ni/Al catalysts Ni/Al2O3 Ni ZnAlOx Co-Ni/Al2O3 Ni–Ce/Al2O3 Ni/CeO2-Al2O3 Ni/CeO2–ZrO2 Pt-Ni/CeO2Al2O3 Rh-Ni/CeO2Al2O3 Ni/La2O3-αAl2O3 Ni- Fe K/dolomite Ni/MCM-41

Reaction temperature (
C) 800 800 850 800 800 800 650 850 800 650 900 650 800 650

Gas yield (%) 81 47 51 52.9 54 48.6 77 53 48 68 71 82 70 82

References Xu et al. (2010) Wang (2013) Dong et al. (2017) Nahil et al. (2013) Nahil et al. (2013) Nahil et al. (2013) Bimbela et al. (2013) Wu and Williams (2011a) Ren et al. (2019) Nishikawa et al. (2008) Fu et al. (2014) Efika et al. (2012) Feng et al. (2017) Efika et al. (2012)

650

83

Efika et al. (2012)

700 800

96 78.12

800

51

Remiro et al. (2013) Chaiprasert and Vitidsant (2009) Wu et al. (2013)

conversion capability of CeO2/SiO2 supported catalyst increases in the order Pd < Pt < Rh. A significant variation in the reforming potential of catalysts can be achieved by combining different catalysts in various proportions and permutations. As the most effective tar reforming catalyst prone to easy deactivation, nickel is combined with other metal and mineral compounds to enhance its properties. The impact of various combinations with Ni catalyst is emphasized in Table 9.9. The addition of magnesium to nickel improves the mechanical strength, attrition resistance, and porous structure of the catalyst considerably, with a modest reduction of 14% in gas yield (Sutton et al. 2001). The oxides of Mg improve the catalyst stability by increasing the metal dispersion of Ni and thereby evading the nickel aggregation. A substantial reduction in coke formation is detected with improved water absorptivity and OH surface mobility by adding magnesium (Wu and Williams 2011a). The compounds of Mg find its application as a CO2 sorbent despite being used as primary and secondary catalysts (Arnold and Hill 2019). Dolomite is a potential mineral source that can practically incorporate Mg with Ni to form catalyst combinations with the aforementioned merits. Aluminum and aluminum oxides are the most used catalytic support for Ni due to their excellent steam reforming reactions and initial catalytic activities amidst a wide range of impurities. Rapid deactivation faced by Al2O3 can be retarded by keeping

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the catalytic weight to biomass flow ratio higher than 0.65. While NiAl2O3 catalyst with 33% Ni shows high gas yield and tar reforming stability, Ni-Al catalyst with 28% Ni exhibits better H2 production and reaction time (Wu and Williams 2011a). Further improvement in catalytic activities of Al can be achieved by supporting it with alkaline earth oxides, natural mineral catalysts, rare metal oxides, etc. An excellent tar conversion of more than 99% with higher gas quality can be attained by supporting Ni/Al with LaO3 (Zhang et al. 2018a, b). The presence of Mg in Ni/Al improves the catalytic resistance to attrition and sintering. However, K has only a feeble effect on catalytic activity compared to Mg. Also, oxides of La and Co can significantly enhance the stability of the catalyst along with increased carbon conversion, reduced coke formation, and a higher syngas yield when used as Ni/Al support (Wu and Williams 2011b). A tremendous improvement in H2 yield with higher tar conversion and coking resistance can be attained by adding Ce to Ni/Al catalysts (Nishikawa et al. 2008). Even if the addition of noble metals like Pt, Ru, Ir, etc., significantly contributes towards the stability of the nickel sites and reduction in coke formation, the effect of Pt is commendable over the others (Wu and Williams 2011a). In general, the oxides of Zr and Si improve H2 yield from gasification without appreciable tar reduction. However, there it with Ni catalyst inhibits coke formation. Similarly, Ca with Ni imparts increased H2 yield, CO selectivity, and decreased CO2 selectivity (Inaba et al. 2006). On the other hand, the presence of Fe in Ni catalyst improves the WSG reaction and hence the gas yield. The addition of dolomite with Ni shows excellent catalytic activity and stability for tar reforming. Among the various Ni/dolomite variants, the catalyst with 15% Ni exhibits the optimum tar reforming (Srinakruang et al. 2006). The coking resistance of Ni/dolomite was found to be superior to Ni/Al2O3 and Ni/SiO2 (Chaiprasert and Vitidsant 2009). In addition to choking resistance, the inhibition of sulfur poisoning makes Ni/dolomite unique among the other catalysts. Furthermore, MgO and CaO in calcined dolomite can act as potential CO2 sorbent along with its excellent tar reformation. The well-defined porous structure, significant surface area, and high thermal stability of zeolite improve the reforming and thermal cracking of hydrocarbons when used with Ni. Zeolite support in Ni catalyst reduces the tar content with the penalty of coke deposit. The coke formation owing to the acid nature of the catalyst can be inhibited by introducing metals like Ce, Mg, etc. (Wu and Williams 2011a). A comparative assessment of commercial nickel catalysts on tar reforming is demonstrated in Table 9.10. In most commercial catalysts, the Ni O content is maintained below 25% to achieve better tar reforming. In addition to the oxides of Mg, Ca, Si, and K, Al2O3 has a prominent fraction in all commercial nickel catalysts. Along with the percentage fraction of the various constituents, reaction temperature also influences the tar conversion rate. The commercial catalysts experience good stability at a higher temperature which can be prolonged to around 60 h (Caballero et al. 1998). Moreover, the coke removal rate in this catalyst is higher than the coke formation rate at high temperatures. Furthermore, using steam or CO2 gasification, deactivated catalysts can be regenerated with much ease (Wang 2013).

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Table 9.10 Tar conversion of commercial nickel-based catalysts Reaction temperature (
C) 785–850

Tar conversion (%) 98–99

660–800

89–99

780–840

95–100

Corella et al. (2004)

(22% NiO-11% MgO-13% CaO-26% Al2O3- 16%SiO2-7% K2O) (12–15% NiO->70% Al2O3)

700–875

73–100

Corella et al. (2004)

785

88–97

(15% NiO-85% K2O)

785–800

92

Caballero et al. (1998) Caballero et al. (1998)

(15%NiO-85% MgAl2O4)

690–780

99

(14% NiO-13% CaO-73%Al2O3)

850–900

99

(14% NiO- 10% CaO-65% Al2O3)

850–900

99

14% NiO-6%CaO-76%Al2O3

650–900

99

Catalyst BASF—G125/1

Composition (25% NiO- 8% CaO-66%Al2O31% K2O)

BASF G1-50

(20% NiO- 11% MgO-16% CaO-32%Al2O3-7% K2O – 32% MgAl2O3) (15% NiO-0.1% SiO2-85% K2O)

Haldor Topsoe—R67 ICI Katalco— 46-1 BASF—G125S Haldor Topsoe— RKS-1 Haldor Topsoe—R67-7H Sud Chemie— G-90LDP Sud Chemie— G-90EW Sud Chemie— G-90B

Reference Chan and Tanksale (2014b) Corella et al. (2004)

Mullen and Boateng (2008) Pfeifer and Hofbauer (2008) Pfeifer and Hofbauer (2008) Pfeifer and Hofbauer (2008

Pertaining to the aforementioned comparison, it can be presumed that Ni, the best tar reforming catalyst under dry and steam atmosphere owing to its excellent catalytic activity towards tar reduction at low cost, is the most accepted catalyst in biomass gasification. But, rapid catalytic activity loss due to carbon deposition confines its application in biomass gasification. Even though doping Ni catalysts with alkaline metals, noble metals or dolomite can influence its overall catalytic performance, agglomeration of alkali metals and sulfur poisoning of noble metals enables the coke resistant dolomite to the more suitable support for Ni catalysts. Thus in a nickel dolomite combination catalyst, the low methane reforming of dolomite and rapid coking of Ni are mutually resolved due to the superior methane conversion and coke resistance of Ni and dolomite, respectively. Additionally, the CaO content in dolomite acts as a sorbent for noncombustible CO2 there by improving the syngas quality. So a combination of Ni and dolomite can perform as an excellent tar reformer and CO2 sorbent. Owing to imparting ample

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value-addition to syngas, Ni/dolomite is supposed to be the better choice among the biomass gasifying catalysts, which can be in the mixed or impregnated form (Di Felice et al. 2009; Xu et al. 2010).

9.10

Catalytic and Deactivation Mechanisms

Catalysts enhance the quality of syngas from gasification by reforming higher hydrocarbons and CH4 into high-valued gas constituents without contributing any pollutants (Gao et al. 2020). This is mainly achieved by reducing the activation energy of reforming reactions, thereby lowering the consumption of gasifying medium. The effectiveness of a catalyst rests on its structure, elemental constitution, reaction mechanism, deactivation resistance, and stability. The catalytic cracking mechanism for tar is a series of (Ren et al. 2019). • Dissociation of hydrocarbons and CH4, and absorption on the metal site. • Dehydrogenation of hydrocarbons and CH4. • Oxidation of hydrocarbon through OH radicals derived from hydroxylation of water. • Production of syngas, CH4, and light hydrocarbons.

9.10.1

Catalytic Mechanism

Due to the inherent intricacy of tar, the tar cracking mechanism may be proposed by considering it from two perspectives (i) as real tar and (2) as a tar model represented by toluene.

9.10.1.1

Reforming Mechanism of Real Tar Using Char Catalyst

Due to the concurrent involvement of acid, alkali, homogeneous and heterogeneous reactions, and catalytic and deactivation effects, the tar cracking mechanism is relatively perplexed. Deposition, dehydrogenation with soot generation, and soot gasification are the main reaction routes involved in the tar cracking mechanism with char, where the alkali-alkaline earth metal variants (AAEM) in the char influence the soot generation rate. In the tar cracking reactions, no further aromatics are formed from benzene and naphthalene, whereas lighter aromatics are formed from alkylatomic compounds. The tar components adsorbed at the char active sites, undergo polymerization, and release gaseous product leaving soot. The H/O/OH-free radicals formed from the dissociated steam and carbon dioxide are adsorbed on the char active sites to initiate tar reforming. The catalytic tar decomposition is further enhanced with additional free radicals from the electron

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Fig. 9.11 Bio-char enabled tar cracking mechanism at 800
C (Zeng et al. 2020)

pair shift towards small tar molecules, where H2O and CO2 dissociate the carbon bonds in smaller aromatic rings. The vacant sites resulting from this bond dissociation are again occupied by H/O/OH radicals, thus allowing many radicals into the char carbon structure. The AAEM species is bonded with the carbon element in the char through oxygen element forming AAEM-Oxygen bond (e.g., K-O bond shown). When tar is adsorbed, the AAEM-Oxygen bond is dissociated, due to the gasification reactions between H2O, CO2, and carbon, and the resulting AAEM species move to a solid-gas boundary, occupying the active sites on the fragmented tar. This AAEM species is further replaced by H/O/OH radicals thereby converting tar to lighter hydrocarbons and gas as shown in Fig. 9.11.

9.10.1.2

Steam Reforming Mechanism of Tar Model (Toluene)

In this section, the mechanism is discussed by representing biomass tar as toluene with LSNFO perovskite catalyst (Oemar et al. 2012). The mechanism is explained in three steps, namely (i) adsorption, (2) reaction, and (3) desorption as shown in Fig. 9.12. During adsorption, water present on the support site is dissociated to form adsorbed H and OH. This OH is then split to form adsorbed H and O, replacing the existing O in the support site. On the metal site, toluene (C7H8) is decomposed into adsorbed CH2, and benzene (C6H6), followed by decomposition of benzene to C2H2. Subsequently, these CH2 and C2H2 react with adsorbed oxygen on the support site to form CH2O and C2H2O groups. The CH2O gets further decomposed into CHO and H, and CHO is combined with available adsorbed oxygen to form H and

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Fig. 9.12 Catalytic reforming mechanism of toluene

CO2. The resulting free radicals of H combine to form hydrogen. On the other hand, C2H2O combines with the adsorbed oxygen to form CH2O and C. Carbon further reacts with oxygen to form CO and CO2. Finally, these resulting H2, CO, and CO2 desorb to form a support site for the gaseous phase.

9.10.2

Deactivation Mechanisms

Deactivation is the loss of catalytic activity over time due to usage. The loss associated with frequent replacement of catalysts can be minimized by using deactivation-resistant catalysts. Thus, a proper understanding of catalyst deactivation is inevitable in designing and developing deactivation inhabitant catalysts. The physical and chemical mechanism leading to the deactivation of tar cracking catalysts can be mainly classified into three, coking, sulfur poisoning, and sintering.

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9.10.2.1

277

Coking or Coke Formation

It is the most common deactivation of catalyst encountered due to the deposition of coke or carbon on the active metal centers and/or pores on the catalyst, thereby reducing the catalytic activity by limiting the interaction between catalyst and reactants (Forzatti and Lietti 1999). Coking is due to the encapsulation of catalysts by carbonaceous deposits originating from the cracking or condensation of hydrocarbon or carbon monoxide and methane. The major reactions that promote coking are (Quitete and Souza 2017; Liu et al. 2019), 2CO $ CðsÞ þ CO2

ð9:10Þ

CO þ H2 $ CðsÞ þ H2 O

ð9:11Þ

CH4 $ CðsÞ þ 2H2 h i m Cn Hm $ nCðsÞ þ H 2 2

ð9:12Þ ð9:13Þ

Amorphous and filamentous are the two variants of carbon deposition on catalysts. The formation of amorphous carbon leads to quick deactivation by encapsulating metal centers whereas, there is no rapid deactivation due to filamentous carbon unless a bulk deposit is formed, leading to reactor clogging (Tan et al. 2020). In general, calcined dolomite is deactivated by an amorphous variant but when calcined dolomite is loaded with Ni, filamentous carbon assumes deactivation. The main factors that influence coking are the size of the active site, temperature, steam to carbon ratio (S/C) and tar composition. Size of metal particles that form the active site influence the deposition of carbon on the catalyst. Active metal sites with larger particle sizes are more vulnerable to carbon deposits. More carbon deposition was reported on Ni sites with bigger particle sizes when incorporated into MCN-41 mesoporous support (Wu et al. 2013). A similar observation is made when toluene is steam reformed with Fe-Ni/Palygorskite (Zou et al. 2018). An increase in temperature decreases coking, for instance, coking was completely absent beyond 750
C (Islam 2020). S/C has a significant role in coking as the addition of steam enhances water gas shift reaction (2) and Boudouard reaction, thereby completely regenerating coked catalyst accompanied with improved gas quality (Quitete and Souza 2017). Improvement in hydrogen yield and reduction in carbon deposit was found in Ni/Al2O3 when steam to carbon ratio is increased (Cao et al. 2020). Tar composition determines the type of carbon formed on catalysts due to coking. Atomic carbon α-C, which readily reacts with steam, is formed by the reforming of phenol whereas, other forms of carbon β-C and γ-C, lesser reactive with steam, is resulted from naphthalene when Ni-Fe alloy combined with olivine is used as a catalyst for reforming (Meng et al. 2018). Also, filamentous and aromatic carbon is generated when the toluene benzene mixture and toluene naphthalene mixture is subjected to reforming in the presence of Ni catalyst.

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Coking of a commercial catalyst can be depressed by selecting suitable catalytic composition and optimal operating conditions. Higher steam to hydrocarbon ratio and improved hydrogen partial pressure are used to restrain carbon-induced catalyst deactivation. The catalytic composition can also restrict coking as ions of alkali metals like potassium when integrated with catalyst catalyzes carbon deposit, thereby facilitating bed catalyst regeneration. Similarly, Pt/Al2O3 can be modified to Pt-Re/Al2O3 to attain prolonged aversion against coking.

9.10.2.2

Sintering

Sintering is a thermally initiated process leading to a reduction in active sites of supported and non-supported catalysts through surface modifications. Particle migration or coalescence and atomic migration are the sintering mechanisms for catalysts based on thermal diffusion of the catalyst constituents. In the former, two particles are transported along with the support, and their collision with each other leads to the formation of a bigger particle. Whereas, in atomic migration, a metal ion discharged by one particle is captured by another. For Ni, particle migration is predominant at temperatures less than 700
C while at higher temperatures particle migration becomes more prevalent. Sintering reduces surface area and porosity and makes the active sites inaccessible for the reactants. The reverse of sintering, termed dispersion, is the phenomenon of reduction of particle growth due to O2 and Cl2 or both, due to the generation of oxides or chlorides of the metal followed by their splitting. Sintering mainly depends on the temperature, atmosphere, carriers, promoters, and porosity (Bartholomew 2001; Morris and Bartholomew 2015). The sintering rate escalates exponentially with temperature notably in the presence of steam resulting in coarser and diverse particle sizes (Ochoa et al. 2018). Higher calcination and reaction temperatures favor thermal-driven metal growth and agglomeration. H2 atmosphere promotes sintering in base metals whereas both O2 and H2 support the same in noble metal catalysts. Lower thermal stability and weak metal-carrier interaction promote the sintering mechanism. The thermal stability of metal with carrier improves in the order carbon < silicon dioxide < aluminum oxide. Unsupported CuO is subjected to sintering compared to supported CuO used for syngas reforming at 800
C (Huang et al. 2013). Addition of promoters that increase (e.g., fluorine, lead, sulfur, oxides of barium, calcium) and decreases (oxygen, oxides of Ce, Ba, Ca) the metal mobility reduces sintering. The possibility of sintering impends when the pore size is comparable with that of metal crystallites. Thus, the tendency of catalyst sintering can be minimized by using thermally stable catalyst composition, avoiding metal-carrier interactions, proper choice of atmosphere and promoters.

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9.10.2.3

279

Sulfur Poisoning

In general, poisoning is the degradation is catalytic activity resulting from firm chemical adsorption of impurities to the active metal sites from the feed stream. During thermochemical processing of biomass, the sulfur contained in the feedstock is converted to hydrogen sulfide which in turn converted to metal sulfide at the active metal sites. Consequently, the catalyst is deactivated as the active metal is encapsulated by inert metal sulfide, leading to deactivation due to sulfidation. Moreover, the lower thermal stability and better mobility of metal sulfides promote sintering. Especially, Ni-based catalysts are prone to this deactivation as hydrogen sulfide formed reacts with the Ni to form nickel sulfide. Ni þ H2 S $ NiS þ H2

ð9:14Þ

The reaction is strongly influenced by temperature, above 900
C nickel irreversibly combines with hydrogen sulfide, whereas at lower temperatures, sulfur forms only polysulfide that can be released easily in a sulfur-free environment (Hepola and Simell 1997). Adding oxygen or steam, or increasing temperature can successfully crack NiS formed, but the use of oxygen or steam leads to the formation of inert NiO or NiSO4, an alternate source of deactivation (Li et al. 2007). Apart from nickel, sulfur poisoning can also inhibit the reactivity of noble and transition metal-based catalysts. Sulfur poisoning reduced the activity of Rh– LaCoO3 catalyst, used for tar reforming of syngas, due to the formation of Rh and Co sulfides (Ammendola et al. 2012). Deactivation of Co-based catalyst was also reported by Gao et al. (2020) due to inert cobalt sulfide formation Co þ H2 S $ CoS þ H2

ð9:15Þ

Ammonia, potassium sulfate, potassium chloride, zinc chloride, and formulations of biomass-derived fly ash are the other compounds that can cause deactivation (Albertazzi et al. 2008). Normally, steps are taken to prevent or minimize sulfur poisoning as the poisoned catalyst is difficult to regenerate. One fundamental step to achieve the same is to subject the feed to suitable pretreatments like hydrodesulfurization or zinc oxide beds to remove sulfur compounds from the stream. Effective reduction of poisoning can be achieved by deploying guard beds ahead of the principal bed.

9.11

Products and Application of Biomass Gasification

Much focus is concentrated on the commercial application of biomass gasification to generate energy due to the renewability, carbon neutrality, and ample availability of biomass. Also, being the only renewable carbon source, biomass will be the future

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feedstock for chemical and transportation fuel production (Boerrigter and Rauch 2006). This section deals with the significant applications and products derived from biomass gasification.

9.11.1

Products

Biomass gasification can provide syngas (termed bio-syngas) as the primary product, which can be further processed to form secondary products such as electricity, chemicals (methanol, ammonia, etc.), liquid, and gaseous biofuels. Liquid biofuels like ethanol, gasoline, and diesel, and gaseous biofuels like methane and hydrogen are produced through direct conversion of syngas. These secondary products find application as industrial chemical feedstocks and energy conversion fuels (Demirbaş 2001; Arcoumanis et al. 2008).

9.11.1.1

Bio-syngas

Being the primary product, the syngas, a mixture of H and CO, mainly finds their application as a chemical and energy feedstock. Syngas is an essential precursor of methane, hydrogen, and valuable chemicals like ammonia, methanol, etc. The production of bio-syngas through steam reforming reaction results in bulk production of hydrogen followed by carbon monoxide, which further undergoes shift reaction to generate additional H2 and CO2. The H2/CO ratio determines the formation of desired products like gasoline, methanol, etc. from syngas. Thus, syngas act as a carrier for power generation and an intermediate product for biofuel production. (Basu 2010; Molino et al. 2016).

9.11.1.2

Heat and Electricity Generation

The producer gas generated from gasification can be consumed in boilers and furnaces to generate heat, in ICE to produce small–medium scale electricity, and in GT to generate small–large scale electricity. In GT, the exhaust gas can be further recovered using a heat recovery steam generator and can be used to generate additional electricity (Bauen 2004).

9.11.1.3

Methanol

Methanol (CH3OH), formed during the catalytic synthesis of syngas, acts as a potential transport and chemical industry feedstock. The primary use of methanol includes making formaldehyde, acetic acid, etc. and converting fats and oils to biodiesel through transesterification (Kumar et al. 2009). The catalytic conversion

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of syngas to methanol is carried out with Cu or Ni-Al-based catalysts. Steam reforming of methane leads to the production of methanol, which can in turn be converted to gasoline (Kumar et al. 2009). CO þ 2 H2 ! CH3 OH

9.11.1.4

ð9:16Þ

Ammonia Synthesis

Ammonia (NH3), synthesized from the combined effect of H2 and N2 on a catalytic bed, is widely used in fertilizers, disinfectants, refrigerants, etc. Iron catalysts with added promoters have a vital role in the synthesis of ammonia. The gas formed from biomass gasification is made to endure the shift reaction followed by a scrubbing and regeneration process before converting to NH3 (Basu 2010). N2 þ 3H2 ! 2NH3

9.11.1.5

ð9:17Þ

Ethanol

Ethanol (C2H6O) is a prominent biomass-derived transport fuel used in the transportation industry. It can be used as a single fuel or is used in spark-ignition engines along with gasoline. Ethanol is produced by the thermochemical conversion of syngas with Rh-based or Cu-based catalysts. But the energy loss occurring during the conversion of syngas to these liquid fuels inversely affects their overall energy conversion efficiency (Kumar et al. 2009; Basu 2010).

9.11.1.6

Gasoline

A hydrocarbon mixture that has a carbon number within the range of 5–11 constitutes gasoline. Using fixed and fluidized beds of zeolite catalysts, gasoline conversion from methanol is exercised through the methanol-to-gasoline (MTG) process. Gasoline is obtained by a double dehydration process with the production of dimethyl ether intermediate (Molino et al. 2016).

9.11.1.7

Diesel

A high-quality (Cetane number > 70) diesel oil, with a similar composition to petrodiesel (produced from petroleum), can be developed by employing the FT process. The diesel thus produced can be used in compression ignition engines along with petrodiesel (Molino et al. 2016).

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Methane

Membrane technology can produce methane in which the compressed syngas is fed to some polymer permeation modules (Molino et al. 2016). The average operating temperature of FT synthesis is between 693 and 893
C. The high temperature of FT also favors methane production, which can be further converted into H2 through catalytic dehydrogenation (Huffman 2011).

9.11.1.9

Hydrogen

Hydrogen can be derived from syngas through catalyzed WGS reactions. Cr supported Fe-based catalysts, and Cu-based catalysts are usually used to enhance the shift reaction (Molino et al. 2016). In automobiles, H2 is employed with zeroemission using fuel cells that can attain high efficiency of about 60%. Moreover, H2 can be used to prepare chemicals and intermediate products like ammonia, methanol, etc. (Kumar et al. 2009).

9.11.2

Applications

The gasification process exercises the conversion of biomass feedstock to a suitable gaseous fuel product. The flexibility of this alternative energy conversion method to produce energy and a variety of fuels with fewer greenhouse emissions tremendously widens its possibilities (Winther et al. 2012). The significant areas of application of biomass gasification include boilers, gas turbines, internal combustion engines, and the synthesis of liquid transport fuels and useful chemicals (Kumar et al. 2009).

9.11.2.1

Boilers

In boilers, energy is harvested from syngas through direct combustion without causing any adverse environmental effects. Even though syngas finds ample applications in power plants, the low overall electric energy efficiency necessitates a cogeneration unit for heat reuse. Similarly, in power plants, biomass gasification products can be burned along with conventional fuels (co-combustion), limiting the weight percentage of the former in the range of 5–15 (Molino et al. 2016).

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Gas Turbines

Syngas finds application in gas turbines (GT) with a relatively higher electrical conversion efficiency of 40% compared to conventional steam cycles. This improved conversion efficiency is attributed to the better combustion characteristics of gaseous fuels. An additional improvement in energy efficiency by up to 60% can be achieved using an additional energy recovery system, and GT is considered the most effective electrical-generation system (Jiang et al. 2016; Ren et al. 2020). However, a rapid turbine blade deterioration due to high-temperature flue gas demands a higher calorific value and appropriate cleaning methodologies.

9.11.2.3

Internal Combustion Engines

The biomass-derived syngas with high calorific value and fewer pollutants are required for the appropriate performance of internal combustion engines. Along with combustion, syngas is also employed for co-combustion with biogas achieving an overall energy conversion efficiency of 45%. But the fruitful application of syngas in engines demands modified fuel supply systems (Shah et al. 2010; Zhang et al. 2018a, b).

9.11.2.4

Fuel Cells

Yet another important application of syngas is its usage in high-energy fuel cells. In fuel cells, the electricity is generated through chemical energy conversion with an energy conversion efficiency greater than 40%. Moreover, direct feeding of fuel cells with syngas or with a mixture of H2 and CH4 adds flexibility (Molino et al. 2016). Furthermore, the noncombustible usage of syngas in high-temperature fuel cells makes the approach sustainable and eco-friendly (Pierobon et al. 2014).

9.11.2.5

Fischer–Tropsch Synthesis

Fischer–Tropsch (FT) synthesis is the catalytic transmutation of syngas to longchain hydrocarbon. Catalytic support for the FT process is provided by iron, cobalt, and nickel-based metals (Basu 2010). The name of the process is coined as a portmanteau of the names of Franz Fischer and Hans Drops who developed the process in 1920. Hydrocarbons of varying molecular weight can be obtained by the reaction between H2 and CO on feeding syngas to the FT reactor. A wide variety of oxygenated compounds with carbon number spanning from 1 to 35 (inclusive of gases and solid waxes) are produced from FT reactions in which hydrocarbons in the span C5–C10 serves as synthetic fuels, and higher range hydrocarbons favor gasoline

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products. The standard operating temperature and pressure of FT reactions are between 200 to 350
C and 20–300 atm. ð2x þ 1ÞH2 þ xCO ðcatalystÞ ! Cx Hð2xþ2Þ þ xH2 O

ð9:18Þ

Where CxH(2x + 2) indicates a hydrocarbon mixture spanning from methane to wax depending on the pressure, catalyst, and temperature maintained in the reaction.

9.12

Recent Advances and Future Perspectives of Catalytic Biomass Gasification

The previous section discussed various catalysts, their classification, mechanisms, and their effect on tar reforming and gas yield. This section deals with the recent trends and future scope in catalytic biomass gasification. Further improvement of catalytic performance through new catalyst configurations and novel technologies are discussed.

9.12.1

Recent Advances

One of the most recent advancements in catalytic biomass gasification is the reactive flash volatilization approach to improve the uneconomical options of fluidized bed gasifiers with a downstream catalytic reactor for tar cleaning or a downstream catalytic reformer with a pyrolysis reactor. A fixed bed gasifier with a significantly higher mass flow rate and carbon space velocity is used in reactive flash volatilization, making it cost-effective on a small scale (Colby et al. 2008). Reactive flash volatilization with Rh-based catalyst is appropriate for cellulose gasification and is a promising approach towards superior syngas production. But the high cost of Rh catalyst confines its application to bench-scale (Chan and Tanksale 2014b). Similarly, a carbonless decomposition of H2 and CO can be attained from a non-volatile feed in less than 50 ms through reactive flash volatilization with Rh-Ce catalyst. The rapid oxidation of decomposition products prevents the condensation reactions and hence the carbon formation. As a result, higher feed conversion with H2 and CO components is achieved with a lower C/O ratio (Salge et al. 2006). Another prospective technology for potential tar reforming is hybrid plasma-catalysis due to its high tar conversion rate and syngas yield. Furthermore, dry reforming of tar to H2 and autothermal reforming (introduction of O2 to promote tar cracking, thereby limiting C deposition) are alternative emerging technologies to replace the conventional steam reforming at the expense of catalytic deactivation and oxygen purification cost, respectively (Sinaei Nobandegani et al. 2016).

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Future Perspectives

With reference to the prior studies, Ni-based catalysts are identified for their outstanding tar reforming capability. The value-addition of Ni catalysts by incorporating noble metals or dolomites is worth researching for further development; Ni-dolomite configurations seek more research attention due to their instant availability and affordability (Wu and Williams 2011a). As a commercially viable process, the gasification of lignocellulose is another area of future scope, where more robust and effective Ni catalysts may take a crucial position (Chan and Tanksale 2014a). Currently, studies are focusing on the improvement of sintering, sulfur poisoning, and coking resistance of catalysts. But the complexity of developing a catalyst by integrating all of these is still challenging. So, synthesizing a multi-functional catalyst with anti-coking, anti-sintering, and anti-poisoning properties is a highly relevant field that requires more attention. Furthermore, poisoning resistance towards halogen and other organic nitrogen species along with catalytic regeneration and recycling is yet another field for further research (Gao et al. 2020). Even though attempts continue to optimize the laboratory-scale reactors and catalysts, scaling-up of the same is more complex. The optimization of space velocity, unstable flow rate, etc. on scaled-up reactors is another area of futuristic interest (Nabgan et al. 2017). Moreover, scaling-up of sorption enhanced steam reforming (SESR) with the integrated sorbent, from laboratory to commercial-scale, also entails much research interest (Gao et al. 2020). Another vital area requiring much attention in the industrial application of biomass gasification is the stability in tar reforming. Sustained tar reforming with simultaneous catalyst regeneration through chemical looping involves relevant consideration (Artetxe et al. 2017). Apart from this, further research should be done in reactor design optimization by incorporating sophisticated techniques employed in bubbling, decoupled triple bed, and countercurrent fluidized bed reactors (Gao et al. 2020). Additionally, elevated temperature steam reforming performance demands further studies on improved reactor material and design. Also, alternative novel improving technologies like dry reforming and autothermal reforming (ATR) are to be explored to their full extent (Sinaei Nobandegani et al. 2016). Another emerging area with promising research opportunities is the mechanism of tar cracking. The actual tar cracking mechanism should be considered with various types of model compounds in varying amounts accounting for their complex structure containing five-ring aromatic hydrocarbons, oxygenates, and polycyclic aromatic hydrocarbons, rather than a single first-order tar model compound (Ren et al. 2019). Along with the catalytic reforming mechanism, the mechanism involving preparation and pre-treatment of catalyst and combined interaction mechanism of metals supports, and promoters are other relevant areas that demand further research (Gao et al. 2020). Furthermore, research focusing on syngas production as a potential secondary feedstock for synthetic fuel production necessitates a one-step chemical fuel synthesis from biomass tar steam reforming with optimized

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parameters and a suitable catalyst, opening the possibilities of a new precinct in biomass gasification (Liu et al. 2019).

9.13

Summary

This chapter deals with various aspects of biomass gasification and the requirement for catalytic producer gas enhancement. Biomass gasification is a complex transmutation of biomass to gaseous fuels and chemical feedstock. The performance of the process is mainly governed by reactor design and key operating parameters such as gasifying agent, type of feedstock, temperature, pressure, and residence time. The fruitful application of producer gas can be ensured only if the undesirable tar content in it is reduced beyond specified limits. Reforming tar using catalysts is identified as the most economical and commercially viable route for achieving the same. The chapter also covers catalytic activation and deactivation mechanisms, and a performance comparison of different catalysts in tar reforming and enhancement of gas yield. It is inferred that the selection of a suitable catalyst not only enhances gas yield through tar reforming but also acts as a CO2 sorbent to sequestrate incombustible CO2, further enhancing producer gas quality. In this view, Ni-dolomite combinations are identified as an economical and potential catalyst that nourishes producer gas through simultaneous attainment of tar reformation and CO2 sorption. Further improvement of catalytic performance through new catalyst configurations and novel technologies are also discussed. Conflict of Interest All the authors declare that they have no competing interests.

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Chapter 10

Recent Advances in Fast Pyrolysis and Oil Upgradation Sameer Ahmad Khan, Dushyant Kumar, Subodh Kumar, Adya Isha, Tinku Casper D’Silva, Ram Chandra, and Virendra Kumar Vijay

Abstract In this chapter, a brief introduction to various pyrolysis processes and a detailed analysis of fast pyrolysis has been explored. A wide variety of feedstocks suitable for the fast pyrolysis process and physicochemical properties of bio-oil has been incorporated. Various types of pyrolyzers that are available in the papers are also mentioned here. Furthermore, enrichment of bio-oil using various upgradation techniques and its physicochemical properties are also discussed. This includes the processes such as steam reforming, catalytic cracking, and supercritical extraction. The application of bio-oil as a fuel requires enhancement in its properties which is achieved using blending with other fuels. Thus, this chapter also strives to explain the recent advances made in bio-oil properties enhancement. Also, a brief analysis of the techno-economic feasibility of bio-oil production and its environmental sustainability is specified. This chapter attempts to give an overview of the whole concept of bio-oil production to its application. Keywords Pyrolysis · Bio-oil · Fast pyrolysis reactor (pyrolyzer) · Upgradation and enhancement of bio-oil · Techno-economics · Environmental sustainability

Abbreviations BCO CE CFBR CR CS CSBR

Bio-crude oil Capillary electrophoresis Circulating fluidized bed reactor Cassava rhizome Cassava stalk Conical spouted bed reactor

S. A. Khan (*) · D. Kumar · S. Kumar · A. Isha · T. C. D’Silva · R. Chandra · V. K. Vijay Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Verma (ed.), Thermochemical and Catalytic Conversion Technologies for Future Biorefineries, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-4312-6_10

297

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dAPCI-TOF MS FTIR FT-NMR GC GPC HAD HDM HDN HDO HDS HPLC HTC HTG HTL ICFBR iPEPICO LC LCA LQIT-dAPCI-MS MBMS MDGC NEV RTP SFCs SOC SPI-TOFMS SVUV-PIMS TAP

10.1

Dopant-assisted atmospheric pressure chemical ionization time-of-flight mass spectroscopy Fourier transform infrared Fourier transform–nuclear magnetic resonance Gas chromatograph Gel permeation chromatography Hydrodearomatization Hydrodemetallation/hydrodemetallization Hydrodenitrogenation Hydrodeoxygenation Hydrodesulfurization High-performance liquid chromatography Hydrothermal carbonization Hydrothermal gasification Hydrothermal liquefaction Internally circulating fluidized bed reactor Imaging photoelectron photo ion coincidence spectroscopy Liquid chromatography Life cycle assessment Linear quadrupole ion trap dopant-assisted atmospheric pressure chemical ionization mass spectroscopy Microreactor for pyrolysis connected with molecular beam mass spectrometer Multidimensional gas chromatography Net energy value Rapid thermal pyrolysis Supercritical fluids Soil organic carbon Single-photon ionization time-of-flight mass spectroscopy Synchrotron-based vacuum ultraviolet photoionization mass spectroscopy Temporal analysis of products

Introduction

As human society progresses towards a new height of technological advancement, the need for per capita energy requirements is also growing tremendously. To meet this necessity of energy, exploration of renewable sources is being highlighted and proposed to fulfill the demands. The dependency of society on non-renewable sources of energy such as fossil fuels will not be accepted in the near future due to scarcity and an increase in population. Also, fossil fuel utilization leads to greenhouse gas emissions that lead to climate change (No 2014). Renewable sources of

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energy are explored to overwhelm petroleum and its related product (Kumar and Badgujar, 2019, Kumar et al., 2022). Lignocellulosic biomass is a promising renewable source that can be considered feedstock for biofuel (Patel and Kumar 2016; Bhardwaj et al. 2021; Kumar and Verma 2021a, b). Biomass is converted to solid, liquid, and gaseous biofuel through chemical, thermal, thermochemical, and biochemical processes. The thermochemical process has some leverage over the biochemical process in terms of reaction time and conversion. In the case of thermochemical, both liquefaction and pyrolysis can produce bio-oil. Still, pyrolysis has an advantage in terms of capital cost, making this process more suitable for converting biomass to bio-oil (Nanduri et al. 2021; Agrawal and Verma 2022; Goswami et al. 2020a, 2021). Biomass can be converted to bio-oil through a fast pyrolysis process. Bio-oil cannot be used directly with petrol or diesel due to its non-miscibility with hydrocarbon, polar, and unstable nature (Patel and Kumar 2016; Goswami et al. 2020b). Other bio-oils, oil from vegetables, and animal wastes cannot be used in raw form in conventional engines. Raw bio-oil obtained through pyrolysis can be used in boilers and furnaces as a replacement fuel instead of fuel oil (Nanduri et al. 2021). But on further upgradation of bio-oil, it can be used as a transportation fuel in diesel engines, fuel for combined heat and power generation, and fuel for gas turbines (No 2014). Depending upon the upgradation technique of pyrolysis oil, it will also result in valuable chemicals (Jacobson et al. 2013).

10.2

Bio-oil

Bio-oil is found in liquid form with dark brown color, which comprises oxygen-rich compound (Mohan et al. 2006; Goswami et al. 2022). Bio-oil produced from pyrolysis of biomass is also referred with various terms in the literature, such as wood pyrolysis oil, wood oil, wood distillates, liquid wood, wood liquid, liquid smoke, bio-crude, bio-crude oil (BCO), bio-fuel oil, pyrolysis liquid, pyrolysis oil, pyrolytic bio-oil, pyrolytic oil, fast pyrolysis bio-oil, fast pyrolysis liquid, biomass fast pyrolysis fuel, biomass pyrolysis liquid, biomass flash pyrolysis liquids (BFPL), pyroligneous tar, and pyroligneous acid (No 2014; Krutof and Hawboldt 2016; Isahak et al. 2012). In this chapter, a liquid produced from pyrolysis of biomass will be termed bio-oil. If bio-oil is from other sources such as plastic, it will be referred to as pyrolysis oil.

10.2.1

Feedstock for Bio-Oil

Bio-oil or pyrolysis oil can be obtained not only from biomass but also from animal waste, vehicular residue, plastic, and paper waste from paper mills each has been discussed in the latter portion of the chapter. Various feedstock categories are cellulose and lignin, bark, wood, nuts and seeds, agricultural wastes/residues,

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algae, forestry residues, grasses, and miscellaneous. After harvest, the biomass is pretreated, such as removing moisture content and reducing particle size, before feeding the reactor to increase the bio-oil yield. There are various studies where co-pyrolysis, instead of using a single feedstock, two or more feedstock were used for the fast pyrolysis process to enhance the bio-oil properties (Lu et al. 1994; Mohan et al. 2006; Abnisa and Wan Daud 2014).

10.2.1.1

Lignin, Cellulose, and Forest Residue

Mohan et al. (2006) reviewed the work of Fullana et al. who performed the pyrolysis of cellulose, lignin, and sewage sludge and analyzed the product obtained on multidimensional gas chromatography (MDGC). In the same review, fast pyrolysis of forest residue and grasses was also discussed. Dalla et al. (2016) produce the bio-oil from forest residue, softwood (Picea abies), and (Eucalyptus sp.). Intermediate, fast, and catalytic fast pyrolysis was performed on these feedstocks. Fast pyrolysis of hardwood and softwood residues shows bio-oil yields of 69% and 70%, respectively.

10.2.1.2

Bark and Wood Trunk

Pinto et al. (2018) performed fast pyrolysis on the bark of Pinus radiata, catechin, and water-insoluble (W-I) tannins from the bark of Pinus radiata and compared the fraction of phenol and organosolv-lignin in pyrolysis oil from the given feed materials. Specific compound catechols show the highest percentage in the pyrolysis oil of W-I tannins. Santos et al. (2018) shows the fast pyrolysis of stump wood and trunk wood from Eucalyptus urograndis with 450–480
C reactor temperature. Combining light and a heavy fraction of pyrolysis oil yield from stump wood and trunk wood is 43.4% (31.6% light fraction and 11.8 heavy fractions) and 47.1 (34.1% light fraction and 13.0 heavy fraction), respectively.

10.2.1.3

Nutshell Waste

Das and Ganesh (2003) showed the yield of pyrolysis oil from cashew nutshell that was in the range of 37% (at 400
C) to the highest of 42% (500–550
C) and then dropped to 36% (600
C) (Das and Ganesh 2003). Another paper related to this study characterized the bio-oil obtained from pyrolysis of cashew nutshell. Shell was first heated up to 175
C, and oil was collected, represented as CO1. After the removal of CO1, it was further pyrolyzed under a vacuum at 500
C to get the maximum quantity of bio-oil, denoted by CO2. Both bio-oil CO1 and CO2 show complete miscibility with diesel (Das et al. 2004).

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A pressed Jatropha carcass was used by Jourabchi et al. (2014) for pyrolysis to study the bio-oil yield in a fixed bed system. The system’s temperature was in the range from 573.15
K to 1073.15
K with a heating rate of 50
K/min in the presence of nitrogen. The maximum yield of bio-oil from this is around 50%.

10.2.1.4

Seeds

Seeds are also used for the production of pyrolysis oil. Onay and Kockar (2003) used rapeseed for the production of bio-oil. A maximum bio-oil yield of 46.7% was found in slow pyrolysis at the temperature of 550
C with a heating rate of 30
C/min without any sweeping gas in the Heinze retort. This bio-oil yield was increased to 51.7% by introducing sweeping gas, i.e., nitrogen, at a flow rate of 100cm3/min. In fast pyrolysis, the maximum bio-oil yield was 63.1% at 550
C at 300
C/min with a sweeping gas flow rate of 100 cm3/min. This yield was increased to 68% at similar conditions with 0.6–1.25 mm particle size and the sweeping gas flowrate was between 100 and 200 cm3/min. On the other hand, flash pyrolysis shows the maximum bio-oil yield of 73% at 600
C with a sweeping gas flow rate of 100 cm3/min.

10.2.1.5

Agricultural Residue

Pattiya and Suttibak (2012) experimented on agricultural residue, i.e., cassava rhizome (CR) and cassava stalk (CS). Fast pyrolysis was done on this waste in a fluidized bed reactor in the presence of nitrogen gas. For CR, the optimum temperature is 475
C, at which bio-oil yield was maximum which was 69.1%, whereas, for CS, the temperature was 469
C, and yield was 61.4%. In another related study, fast pyrolysis in a free-fall reactor in the presence of nitrogen gas on agricultural residue, i.e., sugarcane and cassava residue, was done by Pattiya et al. (2012) for CR and CS maximum bio-yield was 50.3% and 52.3% at 400
C and 450
C, respectively. For sugarcane leaves and sugarcane tops, bio-yield was 53.3% and 44.1% at 400
C and 350
C. Yanik et al. (2007) demonstrated fast pyrolysis on oreganum stalks, corncob, and straw at 500
C in a fluidized bed reactor. Bio-oil yield for oreganum stalks, corncob, and straw was 39%, 41%, and 35%, respectively.

10.2.1.6

Algae

Miao et al. (2004) utilized the microalgae for the fast pyrolysis process. The experiment parameter was as follows: the reaction temperature was 500
C, the heating rate was 600
C/s, sweeping gas (nitrogen) flow rate 0.4 m3/h, and residence time for vapor was 2–3 s. The bio-oil yields from microalgae M. aeruginosa and C. protothecoides were 23.7% and 17.5%, respectively.

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Animal Waste

Even some study shows the production of bio-oil from animal waste. Das et al. (2009) showed the production of bio-oil from the fast pyrolysis of chicken manure. Woo et al. (2015) generate bio-oil from swine manure using the fast pyrolysis method. Emrah et al. 2019 produced bio-oil, using goat manure as feedstock for the pyrolysis process.

10.2.1.8

Other Feedstocks

Bio-oil is not only produced using biomass or organic waste, but it can also be generated using industrial waste, plastic, and other sources of feedstock. Papermill residue is also used to generate pyrolysis oil using a fast pyrolysis process, as mentioned by Charusiri (2015). A comparative study on automobile shredder residues (ASR) was carried out after processing it through conventional and fast pyrolysis (Zolezzi et al. 2004). This study shows that fast pyrolysis of ASR produces a high yield of pyrolysis oil, i.e., 58% at 800
C. Wong (2017) performed the distillation of pyrolysis oil produced from plastic wastes. Fast pyrolysis was conducted at 500–800
C in a laboratory to obtain the pyrolysis oil. Prurapark et al. (2020) explored the effect of temperature on pyrolysis oil generated from HDPE (highdensity polyethylene) and PET (polyethylene terephthalate). In the case of HDPE, the highest pyrolysis oil yield was seen at 450
C, i.e., 40.5 L of pyrolysis oil from 100 kg of HDPE. All the above feedstocks are suitable to produce bio-oil or pyrolysis oil. This also shows that feedstock for fast pyrolysis is not limited to just biomass.

10.2.2

Bio-oil from Pyrolysis

The pyrolysis process is done in an oxygen-free environment at a temperature range of 200
C–900
C. Pyrolysis process depends upon the thermal pretreatments, peak temperature, heating rate, carbonization particle size, and residence time. Pyrolysis can be of a slow, mild, and fast nature. In all cases, biomass goes under thermal decomposition to give biochar, bio-oil, and syngas [C1–C2 hydrocarbon, hydrogen, carbon mono oxide, carbon dioxide] (Baghel et al., 2022; Sakhiya et al., 2020). Pyrolysis products are the transformation of biomass components such as hemicellulose, cellulose, and lignin at various temperature ranges as shown in Fig. 10.1. There are various types of pyrolysis and related processes that mainly produce pyrolysis oil are mentioned in Table 10.1. (Cha et al. 2016).

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Fig. 10.1 Temperature range for the breakdown of biomass components (Zaman et al. 2017)

10.2.2.1

Slow Pyrolysis

Slow pyrolysis, this process aims to produce high carbon content solid product. The residence time can vary from minute to hour; peak temperature can be in the range of 200
C–600
C, and the heating rate is generally less than 50
C/min, and whole process occurs in an inert environment. Generally, the inert environment is created using nitrogen gas (Antal 2003; Cha et al. 2016).

10.2.2.2

Steam Pyrolysis and Vacuum Pyrolysis

Commonly, nitrogen gas is passed through a pyrolysis reactor to establish an inert environment, also known as sweeping gas. If this nitrogen is replaced with steam, the process is modified from slow pyrolysis to steam pyrolysis. In comparison, steam pyrolysis gives more bio-oil yield than slow pyrolysis (Özbay et al. 2008). When slow pyrolysis is performed under the vacuum, then this process is modified to vacuum pyrolysis, which can also be used to produce bio-oil. In steam and vacuum pyrolysis, fundamental parameters such as heating rate, residence time, and peak temperature are similar to slow pyrolysis (Carrier et al. 2011; Özbay et al. 2008).

Above atmospheric Above atmospheric

Atmospheric Atmospheric

With hydrogen

With methane

With vacuum

With water

With water or methanol

With water

Oxygen-free environment

Oxygen-free or in a nitrogen environment Oxygen-free or in nitrogen environment With hydrogen

With vacuum

Hydro-pyrolysis

Methano-pyrolysis

Vacuum pyrolysis

Hydrothermal carbonization Hydrothermal liquefaction

Hydrothermal gasification

Torrefaction (Mild pyrolysis) Fast pyrolysis

Flash-hydro-pyrolysis

Vacuum flash pyrolysis

Flash pyrolysis

Atmospheric

With steam

Below Atmospheric

Above atmospheric

Atmospheric

Above atmospheric

Below atmospheric

Above atmospheric

Above atmospheric

Pressure Atmospheric

Process variation/Environment With nitrogen

Process Slow pyrolysis (conventional) Steam pyrolysis

Slow Slow Slow Very high Very high High Medium

>400
C 200–300
C 650
C 1000
C